* Add .gitignore.
Adjust configuration for gcc/libgcc.

* Gcc will now build libgcc.a, libgcc_eh.a and libgcc_s.so.
* Activate thread support for libgcc.
* Let libgcc use ELF dwarf2 unwinding on Haiku.
* Activate ELF symbol versioning on Haiku.
* Activate use of __cxa_atexit
* Drop fragments of Haiku support for mipsel
Set missing executable flag on libgcc/configure.
Fix configuration of libstdc++.

* The configure script for libstdc++ contained a couple of duplicate
  'haiku*' case entries, where the first would shadow the second.
* Correct several incorrect libtool definitions and paths.
Use crtbeginS.o and crtendS.o for everything.

* The glue code files crtbegin.o and crtend.o don't cleanup globals
  properly, only the variants with the S suffix do. As executables are
  shared on Haiku, we use crtbeginS.o and crtendS.o for those, too.

This fixes crashes on shutdown of Haiku servers that load add-ons (e.g.
Print Server and Media Add-On Server).
Apply shared-library-related fixes to libtool.m4, too.
Explicitly link libgcc and cleanup search paths.

* Adjust the libgcc spec to actually link against libgcc explicitly
  (it is no longer included in libroot.so on anything except x86_gcc2).
* Apply --as-needed when linking against libgcc_s.so
* Drop search paths that no longer exist (/boot/common/...).
Let gcc2 use crtbeginS.o and crtendS.o, too.

* I doubt that this makes much of a difference, but it avoids yet
  another indirection in Haiku's build system.