libatomic: x86_64: Always try ifunc

We used to skip ifunc check when CX16 is available.  But now we use
CX16+AVX+Intel/AMD for the "perfect" 16b load implementation, so CX16
alone is not a sufficient reason not to use ifunc (see PR104688).

This causes a subtle and annoying issue: when GCC is built with a
higher -march= setting in CFLAGS_FOR_TARGET, ifunc is disabled and
the worst (locked) implementation of __atomic_load_16 is always used.

There seems no good way to check if the CPU is Intel or AMD from
the built-in macros (maybe we can check every known model like __skylake,
__bdver2, ..., but it will be very error-prune and require an update
whenever we add the support for a new x86 model).  The best thing we can
do seems "always try ifunc" here.

libatomic/ChangeLog:

	* configure.tgt: For x86_64, always set try_ifunc=yes.