cmake: test for 16-byte atomic support on mips also

cmake: test for 16-byte atomic support on mips also

Origin: upstream, https://github.com/ceph/ceph/commit/709a77f22010f03aee4a4c0ab930588944cb4a58
Last-Update: 2021-11-24

it's reported that a mips64el build host is able to pass the test of
CheckCxxAtomic without linking against libatomic, while librbd.so
fails to link due to failures like

/usr/bin/ld: ../../../lib/librbd.so.1.16.0: undefined reference to `__atomic_store_16'
/usr/bin/ld: ../../../lib/librbd.so.1.16.0: undefined reference to `__atomic_load_16'
/usr/bin/ld: ../../../lib/librbd.so.1.16.0: undefined reference to `__atomic_compare_exchange_16'

so we have to check the existence of __atomic_load_16 instruction on
mips architecture.

Gbp-Pq: Name cmake-test-for-16-bytes-atomic-support-on-mips-also.patch