[arch][arm-m] optimize the arch_interrupt_save routine a bit for cortex-m
[arch][arm-m] optimize the arch_interrupt_save routine a bit for cortex-m

From looking at the dissassembly the compiler wasn't doing a good job
with the overly complicated version shared with the larger arm cores.
Simplify it by recognizing that you can save the state directly from
PRIMASK and restore it more simply.

travisg/lk,travisg/lk,littlekernel/lk,hollanderic/lkstuff,hollanderic/lkstuff,hollanderic/lkstuff,littlekernel/lk,travisg/lk,hollanderic/lkstuff,littlekernel/lk,littlekernel/lk,travisg/lk