Make sure no code in ld.so uses xmm/ymm registers on x86-64.

This patch introduces a test to make sure no function modifies the
xmm/ymm registers.  With the exception of the auditing functions.

The test is probably too pessimistic.  All code linked into ld.so
is checked.  Perhaps at some point the callgraph starting from
_dl_fixup and _dl_profile_fixup is checked and we can start using
faster SSE-using functions in parts of ld.so.