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

llvm-svn: 102400