test_libFLAC.vcproj : Add missing files.

Patch-from: lvqcl <lvqcl.mail@gmail.com>
libFLAC/cpu.c : Bundle of minor fixes.

Includes:

* Replace 'CALLBACK' with 'WINAPI' because the signature of an unhandled
  exception filter uses 'WINAPI'.
* Improvements to OS SSE testing code.
* Improvements to GCC asm code.
* Comment fixes.

Patch-from: lvqcl <lvqcl.mail@gmail.com>