ARM: use AAPCS-style prologues for embedded MachO.

ARM: use AAPCS-style prologues for embedded MachO.

Darwin prologues save their GPRs in two stages: a narrow push of r0-r7 & lr,
followed by a wide push of the remaining registers if there are any. AAPCS uses
a single push.w instruction.

It turns out that, on average, enough registers get pushed that code is smaller
in the AAPCS prologue, which is a nice property for M-class programmers. They
also have other options available for back-traces, so can hopefully deal with
the fact that FP & LR aren't adjacent in memory.

rdar://problem/15909583

git-svn-id: https://llvm.org/svn/llvm-project/llvm/trunk@209895 91177308-0d34-0410-b5e6-96231b3b80d8