arch: arm: cortex_m: Apply clang-format on cortex_m related code

This commit updates cortex_m related code to align it with the rules from
.clang-format. This is done to simplify future changes in these files as
we are about to implement use_switch support.

Some rules conflict with checkpatch and therefore some small part of the
code locally disable clang-format.

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