Enable ARM base pointer when calling functions with large arguments.

When an outgoing call takes more than 2k of arguments on the stack, we
don't allocate that call frame in the prolog, but adjust the stack
pointer immediately before the call instead.

This causes problems with the emergency spill slot because PEI can't
track stack pointer adjustments on the second pass, and if the outgoing
arguments are too big, SP can't be used to reach the emergency spill
slot at all.

Work around these problems by ensuring there is a base or frame pointer
that can be used to access the emergency spill slot.

<rdar://problem/10917166>

llvm-svn: 151604