arch: arm: cortex_m: use local label in ASM code

Regular label are exported in the object file and cause gdb to consider
them as function start. Local labels on the other hand are not exported.
For example, using `disassemble z_arm_pendsv` after this change will
disassemble the whole function rather than stop at the first branch.

Signed-off-by: Wilfried Chauveau <wilfried.chauveau@arm.com>