Optimize stack overflow handling.

We now subtract the frame size from the stack pointer for methods
which have a frame smaller than a certain size. Also changed code to
use slow paths instead of launchpads.

Delete kStackOverflow launchpad since it is no longer needed.

ARM optimizations:
One less move per stack overflow check (without fault handler for
stack overflows). Use ldr pc instead of ldr r12, b r12.
Code size (boot.oat):
Before: 58405348
After: 57803236

TODO: X86 doesn't have the case for large frames. This could case an
incoming signal to go past the end of the stack (unlikely however).

Change-Id: Ie3a5635cd6fb09de27960e1f8cee45bfae38fb33