meson: ensure -mcx16 is passed when detecting ATOMIC128

meson: ensure -mcx16 is passed when detecting ATOMIC128

Moving -mcx16 out of CPU_CFLAGS caused the detection of ATOMIC128 to
fail, because flags have to be specified by hand in cc.compiles and
cc.links invocations (why oh why??).

Ensure that these tests enable all the instruction set extensions that
will be used to build the emulators.

Fixes: c2bf2ccb266 ("configure: move -mcx16 flag out of CPU_CFLAGS", 2024-05-24)
Reported-by: Alex Bennée <alex.bennee@linaro.org>
Reviewed-by: Michael Tokarev <mjt@tls.msk.ru>
Reviewed-by: Alex Bennée <alex.bennee@linaro.org>
Tested-by: Alex Bennée <alex.bennee@linaro.org>
Cc: qemu-stable@nongnu.org
Signed-off-by: Paolo Bonzini <pbonzini@redhat.com>
(cherry picked from commit 8db4e0f92e83fd80b6609439440b303ddded7ad8)
Signed-off-by: Michael Tokarev <mjt@tls.msk.ru>