on darwin empty functions need to codegen into something of non-zero length,

on darwin empty functions need to codegen into something of non-zero length,
otherwise labels get incorrectly merged.  We handled this by emitting a
".byte 0", but this isn't correct on thumb/arm targets where the text segment
needs to be a multiple of 2/4 bytes.  Handle this by emitting a noop.  This
is more gross than it should be because arm/ppc are not fully mc'ized yet.

This fixes rdar://7908505

git-svn-id: https://llvm.org/svn/llvm-project/llvm/trunk@102400 91177308-0d34-0410-b5e6-96231b3b80d8