kernel/arm: implement entering userspace

Change-Id: Icf3945db979a8f4444856fc7157649f48b297c29
Reviewed-on: https://review.haiku-os.org/c/haiku/+/5037
Tested-by: Commit checker robot <no-reply+buildbot@haiku-os.org>
Reviewed-by: Fredrik Holmqvist <fredrik.holmqvist@gmail.com>
Reviewed-by: waddlesplash <waddlesplash@gmail.com>

ARM: Add asm_offsets.cpp

This will generate asm_offsets.h which makes our assembly code
easier to maintain by preventing hardcoded offsets for fields within structures.

(copied from X86 and removed the X86 specifics)

ARM: Add asm_offsets.cpp

This will generate asm_offsets.h which makes our assembly code
easier to maintain by preventing hardcoded offsets for fields within structures.

(copied from X86 and removed the X86 specifics)