Atomics: GCC fix for M23 (again)
Atomics: GCC fix for M23 (again)

Add another missing unified syntax directive. Was previously fixed in
03f1ac3ffd, but the same problem was not addressed in the pending PR that
added the `NEWVAL_2OP` assembler.

andcor02/mbed-os,kjbracey-arm/mbed,andcor02/mbed-os,kjbracey-arm/mbed,mbedmicro/mbed,mbedmicro/mbed,andcor02/mbed-os,mbedmicro/mbed,andcor02/mbed-os,andcor02/mbed-os,andcor02/mbed-os,mbedmicro/mbed,mbedmicro/mbed,kjbracey-arm/mbed,kjbracey-arm/mbed