Implement a basic MCCodeEmitter for PPC. This doesn't handle

fixups yet, and doesn't handle actually encoding operand values,
but this is enough for llc -show-mc-encoding to show the base
instruction encoding information, e.g.:

	mflr r0                         ; encoding: [0x7c,0x08,0x02,0xa6]
	stw r0, 8(r1)                   ; encoding: [0x90,0x00,0x00,0x00]
	stwu r1, -64(r1)                ; encoding: [0x94,0x00,0x00,0x00]
Ltmp0:
	lhz r4, 4(r3)                   ; encoding: [0xa0,0x00,0x00,0x00]
	cmplwi cr0, r4, 8               ; encoding: [0x28,0x00,0x00,0x00]
	beq cr0, LBB0_2                 ; encoding: [0x40,0x00,0x00,0x00]

llvm-svn: 119116