Compile fb_test_amd64 only if adx instructions can be compiled

Compile fb_test_amd64 only if adx instructions can be compiled

Note: this just unbreaks the build on avx + non_adx capable systems
(such as gcc farm gcc20).

adx capable system should probably be better handled:
  * ./tests/x86_amd64_features cannot check for adx flag
    (so fb_test_amd64 is run if compiled and system is avx capable, which
     might give problems if gcc/as can compile the test, but the cpu
     cannot execute adx instructions)
  * on an adx capable system, a native run of cpuid tells it is adx capable
    but under valgrind, cpuid reports the valgrind synthetic cpu is not adx
    capable.

git-svn-id: svn://svn.valgrind.org/valgrind/trunk@16401