arm: make signal handlers work

arm: make signal handlers work

A few kernel and calling convention adjustments to make sigsend and
sigreturn work for arm.

	. provide a arch_proc_setcontext for earm in kernel
	. set LR in context of signal handler to provide a proper
	  return address (to __sigreturn)
	. change __sigreturn to retrieve the sigcontext pointer
	  from the sigframe struct and pass it to _sigreturn() in r0

Change-Id: Icd135a70595382c79d11d8dd9876f6a6f1df41f8