Use 32-bit ebp for NaCl64 in a limited case: llvm.frameaddress.

Summary:
Follow up to [x32] "Use ebp/esp as frame and stack pointer":
http://reviews.llvm.org/D4617

In that earlier patch, NaCl64 was made to always use rbp.
That's needed for most cases because rbp should hold a full
64-bit address within the NaCl sandbox so that load/stores
off of rbp don't require sandbox adjustment (zeroing the top
32-bits, then filling those by adding r15).

However, llvm.frameaddress returns a pointer and pointers
are 32-bit for NaCl64. In this case, use ebp instead, which
will make the register copy type check. A similar mechanism
may be needed for llvm.eh.return, but is not added in this change.

Test Plan: test/CodeGen/X86/frameaddr.ll

Reviewers: dschuff, nadav

Subscribers: jfb, llvm-commits

Differential Revision: http://reviews.llvm.org/D6514