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

From Linux 3.17 6ebbf2ce437b33022d30badd49dc94d33ecfa498:

    ARMv6 and greater introduced a new instruction ("bx") which can be used
    to return from function calls.  Recent CPUs perform better when the
    "bx lr" instruction is used rather than the "mov pc, lr" instruction,
    and this sequence is strongly recommended to be used by the ARM
    architecture manual (section A.4.1.1).

Signed-off-by: Park Ju Hyung <qkrwngud825@gmail.com>

Test: No regressions detected
Test: Passes full CTS run

Change-Id: Ie268f9893e3df0f68fbfe82a13f3c7cc5c5909d8
Signed-off-by: Alex Naidis <alex.naidis@linux.com>