[MC][AMDGPU][llvm-objdump] Synthesized local labels in disassembly

1. Add an accessor function to MCSymbolizer to retrieve addresses
   referenced by a symbolizable operand, but not resolved to a symbol.
   That way, the caller can synthesize labels at those addresses and
   then retry disassembling the section.

2. Implement that in AMDGPU -- a failed symbol lookup results in the
   address being added to a vector returned by the new function.

3. Use that in llvm-objdump when using MCSymbolizer (which only happens
   on AMDGPU) and SymbolizeOperands is on.

Differential Revision: https://reviews.llvm.org/D101145

Change-Id: I19087c3bbfece64bad5a56ee88bcc9110d83989e