Cleanup: Fix missing evaluation of HAIKU_NO_LIBSUPC++.

* There's no user of HAIKU_NO_LIBSUPC++ anywhere, so the typo didn't
  have any effect.
Improve IsPackageAvailable()

* When adjusting the package name for the secondary architecture,
  it is unclear where exactly in the package name the architecture
  specifier is. To remedy, we try all possible positions until
  we find the package (or there are no other possibilities).
Add build features gcc_syslibs and gcc_syslibs_devel.

* gcc_syslibs contains the shared libraries that are being built
  as part of gcc, i.e. libgcc_s.so, libsupc++.so and libstdc++.so
* gcc_syslibs_devel contains the static versions of the respective
  libraries plus additionally kernel versions of libgcc.a and
  libsupc++.a (which do not provide support for pthreads or TLS
  and thus do not have any external dependencies). Additionally,
  this build feature contains the c++ and gcc headers from the
  non-legacy compiler.
Add SystemLibraryRules.

* This file contains jam rules for getting the different versions
  of libgcc, libsupc++ and libstdc++ used throughout Haiku's
  build system.
* Additionally, there are rules for accessing the c++ headers
  and the gcc headers.
* These rules are included by Jamrules, but not yet used anywhere.
Define two pseudo targets for the target and build platform.

* The main build rules now cause their targets to depend on the
  platform such that global per-platform intializations can be
  set up by making the platform pseudo target depend on the
  target returned by the initialization rule.
* When using the non-legacy compiler, use the c++ and gcc headers
  provided in the gcc_syslibs_devel build feature for building Haiku.
* Simplify declaration of c++ and gcc headers for the legacy compiler -
  in the end we always use the ones living by our source tree anyway.
* Fix a couple of missing local declarations for jam variables, which
  were necessary to avoid a build problem with strace. There are
  probably more bugs like these hiding in our build system files, but
  I'm saving the fix for those to the next commit.
* Add new gcc packages to the HaikuPorts (x86*-)repositories.
Add more missing 'local's to variable declarations.
Switch to using crtbeginS.o and crtendS.o unconditionally.
Use libstdc++, libsupc++ and libgcc from gcc_syslibs.

* Instead of faking libstdc++.so from libstdc++.a, use libstdc++.so
  from the gcc_syslibs build feature for everything except x86_gcc2.
* Use libgcc_s.so from the gcc_syslibs build feature for everything but
  x86_gcc2 (which still carries libgcc as part of libroot.so).
* Drop filtering of libgcc objects for libroot, as that is no longer
  necessary since we're only using libgcc-as-single-object for libroot
  with x86_gcc2, where the filtered object file doesn't exist. Should
  the objects that used to be filtered cause any problems as part of
  libgcc_s.so, we can always filter them as part of the gcc build.
* Use libsupc++.so from the gcc_syslibs build feature for everything but
  x86_gcc2.
* Adjust all Jamfiles accordingly.
* Deactivate building of faked libstdc++.so for non-x86-gcc2. For
  x86_gcc2, we still build libstdc++.so from the sources in the Haiku
  source tree as part of the Haiku build .
* Put gcc_syslibs package onto the image, when needed.
Let runtime_loader ignore RT_RELRO program headers.

* The new libstdc++.so contains program headers of type PT_RELRO (for
  making segments read-only after relocation). While the actual feature
  has not been implemented, the runtime_loader should now silently
  accept (and ignore) those program headers.
Cleanup build system and configuration.

* Drop unused variables from build system that refer to the system
  libraries.
* Drop unused lists of libgcc objects.
* Drop no longer used variables from configuration script.
* Remove no longer needed building of kernel-libgcc and -libsupc++ from
  build_cross_tools_gcc4, only the boot-specific (32-bit) libs are
  built for a x86_64 target.
* Explicitly disable threads and TLS support when building the cross
  compiler, as the only libraries that are used by Haiku's build system
  is the 32-bit libgcc and libsupc++ for the bootloader on x86_64 (and
  for that neither is wanted).
Drop Jamfile for building non-legacy libstdc++.so.

* We no longer fake it by converting the static lib into a shared one.
Add support for building <stubbed>libroot.so.

* <stubbed>libroot.so is a shared library which contains all the symbols
  from libroot, but without any code. This library will be required by
  the (to be introduced) stage0 of the bootstrap process, in order to
  be able to link the shared gcc syslibs (libstdc++.so, libsupc++.so
  and libgcc_s.so).
Cleanup with respect to headers rules.

* Move specialcasing of gcc2 from HeadersRules into the respective rule
  in SystemLibraryRules.
Set default of HAIKU_CONTINUOUS_INTEGRATION_BUILD.
Only add Webpositive to image if it is available.
Add pseudo target for bootstrap_stage0 platform.
Fix missing symbols in <stubbed>libroot.so.

* A couple of symbols reuse the same address (via weak symbols), so
  we adjust generate_stubs.py accordingly.
Optionally build glue code for bootstrap_stage0 platform.
Simplify handling of package names.

* No longer apply somewhat crude mechanism for converting a package name
  into a corresponding port name - haikuporter by now supports package
  names directly.

This fixes a problem when trying to pass 'gcc_syslibs_devel' to
haikuporter, which only ever saw 'gcc_syslibs'.