Add a test that verifies modifying SP (REG_R13) in the saved register
context is honored on signal return. The test pushes values onto the
stack, triggers SIGALRM, and in the handler advances SP to skip a
value, then verifies the correct value is popped after signal return.
This exercises the SP relocation fix in arm_sigdeliver.c for ARMv7-M
and ARMv8-M architectures.
Signed-off-by: Andrew Au <cshung@gmail.com>
