[0/3] AVX branch merge to trunk

[H.J. Lu]

Hi,

We are submmiting patches to merge AVX branch to trunk. I synced
AVX branch with trunk at revision 139377.  There are no regressions
on Linux/ia32 and Linux/Intel64. Both 32bit and 64bit of SPEC CPU
2000/2006 ran correctly under AVX SDE, compiled with

-O2 -mavx -Wa,-msse-check=error -mfpmath=sse -ffast-math

I believe it is ready for merging with trunk.  We break it down to 3
patches:

[1/3] Update x86 backend.
[2/3] Update document.
[3/3] New testscases.

Thanks.