This fixes crashes that would occur when attempting to use
libjpeg-turbo's AVX2 extensions on older O/S's (such as Windows XP or
RHEL 5.) Even if the CPU supports AVX2, the O/S has to also support
saving/restoring YMM registers when switching contexts.
The SIMD glue code has gotten a bit #ifdef heavy so clean it up by having
one file for each possible SIMD arch. This also allows a simplification of
the x86_64 code as SSE/SSE2 is always known to exist on that arch.
git-svn-id: svn+ssh://svn.code.sf.net/p/libjpeg-turbo/code/trunk@49 632fc199-4ca6-4c93-a231-07263d6284db
