Atomics: GCC fix for M23 (ARMv8-M baseline)
Atomics: GCC fix for M23 (ARMv8-M baseline)

Add unified syntax directives to make the atomic assembler work when GCC
is building for M23.

GCC actually uses unified syntax when compiling C code, but puts
`.syntax divided` before each piece of inline assembly when targetting
Thumb-1 type devices like M0 and M23 for backwards compatibility. We can
overcome this with our own `.syntax unified`.  The command-line option
`-masm-syntax-unified` intended to override this globally has been
broken from GCC 6 to 8.0.

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