configure: move -mcx16 flag out of CPU_CFLAGS

The point of CPU_CFLAGS is really just to select the appropriate multilib,
for example for library linking tests, and -mcx16 is not needed for
that purpose.

Furthermore, if -mcx16 is part of QEMU's choice of a basic x86_64
instruction set, it should be applied to cross-compiled x86_64 code too;
it is plausible that tests/tcg would want to cover cmpxchg16b as well,
for example.  In the end this makes just as much sense as a per sub-build
tweak, so move the flag to meson.build and cross_cc_cflags_x86_64.

This leaves out contrib/plugins, which would fail when attempting to use
__sync_val_compare_and_swap_16 (note it does not do yet); while minor,
this *is* a disadvantage of this change.  But building contrib/plugins
with a Makefile instead of meson.build is something self-inflicted just
for the sake of showing that it can be done, and if this kind of papercut
started becoming a problem we could make the directory part of the meson
build.  Until then, we can live with the limitation.

Signed-off-by: Artyom Kunakovsky <artyomkunakovsky@gmail.com>
Message-ID: <20240523051118.29367-1-artyomkunakovsky@gmail.com>
[rewrite commit message, remove from configure. - Paolo]
Signed-off-by: Paolo Bonzini <pbonzini@redhat.com>

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