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

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

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.

Signed-off-by: Kefu Chai <tchaikov@gmail.com>