Fix issue with negative value for lisp::cycles-per-tick.

This causes negative cpu cyles with TIME. Basic issue is that a cpu
frequency of 2.3 GHZ won't fit in an int. Use an unsigned int.

We also take this opportunity to use a rounded value for
clocks-per-tick instead of truncating.  For this particular case the
ratio is actual 68.99 which would truncated to 68. We should probably
use 69 instead.
First cut at removing ppc-darwin-dlshim.c. Not needed anymore because
dlsym works on OSX/ppc like on other ports.

A cross-compile was used with cross-ppc-ppc-darwin.lisp.  I'm not sure
a cross-compile is required, though.

 * code/ppc-vm.lisp:
   * EXTERN-ALIEN-NAME doesn't need to prepend an underscore anymore. 
 * lisp/Config.ppc_darwin:
   * Don't compile/link ppc-darwin-dlshim.c.
 * lisp/os-common.c:
   * Don't prepend underscore for call_into_lisp.
 * tools/cross-scripts/cross-ppc-ppc-darwin.lisp:
   * EXTERN-ALIEN-NAME doesn't need to prepend an underscore anymore.
Unify extern-alien-name for darwin x86 and ppc.

Initial attempt to unify extern-alien-name, primarily for darwin (x86
and ppc). Thus, systems using elf do not prepend an underscore, but
other systems are assumed to prepend an underscore.

This is currently only for darwin/x86.  A cross-compile on darwin x86
is needed.

 * code/x86-vm.lisp:
   * For elf systems, don't prepend an underscore.  For other systems,
     do prepend.
 * compiler/generic/new-genesis.lisp:
   * Fix long-standing bug where we should call EXTERN-ALIEN-NAME to
     get the right name for "resolve_linkage_tramp".
 * lisp/Darwin-os.c:
   * Update os_dlsym to assume the external name is always preceded by
     an underscore.  The lookup for dlysm is done by stripping off the
     leading underscore.
 * lisp/os-common.c:
   * Add EXTERN_ALIEN_NAME macro to create the correct alien name for
     elf and non-elf systems.
   * Use it to generate the correct name to be used for comparison in
     the sanity checks of the linkage table entries.
Prepend underscore in extern-alien-name for darwin/ppc.
On sparc, use EXTERN_ALIEN_NAME too when printing out the error message.
For x86, define FEATURE_ELF if needed.

And this is definitely needed now on Linux at least to define the
right EXTERN_ALIEN_NAME.
For sparc, define FEATURE_ELF because Solaris uses elf.

This is needed on sparc to define the right EXTERN_ALIEN_NAME.
Add boot file for the darwin/x86 cross-compile that is used to fix the
extern-alien-name issue so darwin/x86 matches darwin/ppc.
Add :elf to *features* for sparc/solaris.