share/mk/bsd.cpu.mk: add F16C feature for i386 and amd64 architectures

As discussed in bug 278417, some ports require the F16C instruction set
to compile, but there is no way yet to detect whether the currently
chosen CPUTYPE supports this feature.

Add the feature to the MACHINE_CPU variable, for each processor that
supports it. The list of processors was extracted from clang 18's -dM
output, filtered on the F16C define.

PR: 278417
Reviewed by: brooks, emaste
MFC after: 3 days
Differential Revision: https://reviews.freebsd.org/D44848