Add and commonize encoder support for all immediates.

Stub out some dummy fixups to make things work.

We can now emit fixups like this:
	subl	$20, %esp               ## encoding: [0x83,0xec,A]
                                        ##   fixup A - offset: 2, value: 20, kind: fixup_1byte_imm

Emitting $20 as a single-byte fixup to be later resolved
by the assembler is ridiculous of course (vs just emitting
the byte) but this is a failure of the matcher, which 
should be producing an imm of 20, not an MCExpr of 20.

llvm-svn: 95860