[Bug libitm/53113] Build fails in x86_avx.cc if AVX disabled by -mno-avx but supported by as

[dirtyepic at gentoo dot org]

http://gcc.gnu.org/bugzilla/show_bug.cgi?id=53113

--- Comment #21 from Ryan Hill <dirtyepic at gentoo dot org> ---
Well in practice we've had to have users build GCC with -mno-avx on no less
than three occasions since 4.4 due to compiler bugs on certain chips (usually
newer chips + old releases), so it'd be nice to have it just work.

If x86_avx.cc must be compiled with -mavx then can it be appended after user
CFLAGS?  That should make everyone happy.