[BOLT][NFC] Reduce Target/{AArch64,X86} dependencies

We don't actually depend on entire X86/AArch64 components that pull in CodeGen,
SelectionDAG etc., just the Desc part with opcode and other definitions.

Note that it doesn't decouple BOLT from these components - we still pull in X86
and AArch64 from top-level llvm-bolt dependencies as we use assembler and
disassembler. It's difficult to reduce these as this requires non-trivial
changes to X86/AArch64 components themselves (e.g. moving out AsmPrinter).

Reviewed By: rafauler

Differential Revision: https://reviews.llvm.org/D124206