arch: arm: Separate common thread code

This commit separates thread.c into two source files,
'cortex_a_r/thread.c' and 'cortex_m/thread.c, it also introduces some
changes.

1. Migrate 'thread.c' and 'cortex_m/thread.c'.
2. Migrate 'thread.c' and 'cortex_a_r/thread.c'
3. Remove the 'z_arm_mpu_stack_guard_and_fpu_adjust' function as this is
   obviously written for Cortex-M architecture.
4. Remove the 'z_arm_prepare_switch_to_main' function as this is only
   used by Cortex-M architecture.

Signed-off-by: Huifeng Zhang <Huifeng.Zhang@arm.com>