[M23] Fix CThunk error on Cortex-M23
[M23] Fix CThunk error on Cortex-M23

Cortex-M23 doesn't support ARMv8-M Main Extension and so doesn't support:
ldm  r0, {r0, r1, r2, pc}

Fix it by going Cortex-M0/M0+ way:
ldm  r0, {r0, r1, r2, r3}
bx   r3

betzw/mbed-os,c1728p9/mbed-os,andcor02/mbed-os,c1728p9/mbed-os,andcor02/mbed-os,mbedmicro/mbed,andcor02/mbed-os,mbedmicro/mbed,betzw/mbed-os,betzw/mbed-os,kjbracey-arm/mbed,c1728p9/mbed-os,betzw/mbed-os,andcor02/mbed-os,kjbracey-arm/mbed,betzw/mbed-os,mbedmicro/mbed,c1728p9/mbed-os,mbedmicro/mbed,kjbracey-arm/mbed,c1728p9/mbed-os,kjbracey-arm/mbed,betzw/mbed-os,andcor02/mbed-os,mbedmicro/mbed,andcor02/mbed-os,c1728p9/mbed-os