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>

Author: SgrrZhf

arch/arm/core/CMakeLists.txt

@@ -7,7 +7,6 @@ zephyr_library_sources(
   fatal.c
   nmi.c
   nmi_on_reset.S
-  thread.c
 )
 
 zephyr_library_sources_ifdef(CONFIG_CPP __aeabi_atexit.c)

arch/arm/core/cortex_a_r/CMakeLists.txt

@@ -16,6 +16,7 @@ zephyr_library_sources(
   swap_helper.S
   irq_manage.c
   prep_c.c
+  thread.c
   )
 
 zephyr_library_sources_ifdef(CONFIG_GEN_SW_ISR_TABLE isr_wrapper.S)