linker: convert 'mov pc' instruction to 'bx'

Let's follow Linux 3.17 : https://github.com/torvalds/linux/commit/6ebbf2ce437b33022d30badd49dc94d33ecfa498

The only problem is we don't know if the target is ARMv6+ or not.

ARMv6+ should be fine with this patch.

Signed-off-by: arter97 <qkrwngud825@gmail.com>

Conflicts:
	linker/arch/arm/begin.S

(cherry picked from commit 0685deb5e6b33fbcd532bdb099369981eed9a203)