Add and commonize encoder support for all immediates.

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.

git-svn-id: https://llvm.org/svn/llvm-project/llvm/trunk@95860 91177308-0d34-0410-b5e6-96231b3b80d8