arm: access svc instruction using halfword load in svc_handler

Use of `ldr` triggers unaligned memory access when loading SVC
instruction to r0. This is caused by the fact that SVC is a 16-bit
instruction, hence with a 2 byte offset, we are performing an non-word
aligned access. Prevent this by using `ldrh` to load a halfwords rather
than full words.

Change-Id: Ieae60c2ce86c6cfe15c89627d3a450797ce7e714
Signed-off-by: Maciej Borzecki <maciek.borzecki@gmail.com>