Don't report BMI support when AVX support is missing.

Bug #326113. This is a bit conservative, but it is what the linux kernel
also seems to be doing. If AVX support is missing (because the OS doesn't
save the wide registers), then also don't report supporting BMI.

git-svn-id: svn://svn.valgrind.org/valgrind/trunk@13656 a5019735-40e9-0310-863c-91ae7b9d1cf9
arm-linux only: make unwinding by stack scanning (a nasty hack)
be controllable from the command line.  Fixes (kind of) #289578.


git-svn-id: svn://svn.valgrind.org/valgrind/trunk@13657 a5019735-40e9-0310-863c-91ae7b9d1cf9