[MC] Changes to help improve target specific symbol disassembly

Summary:
This commit slightly modifies the MCDisassembler, and llvm-objdump to
allow targets to also decode entire symbols.

WebAssembly uses the onSymbolStart hook it to decode preludes.
WebAssembly partially disassembles the symbol in its target specific
way; and then falls back to the normal flow of llvm-objdump.

AMDGPU needs it to decode kernel descriptors entirely, and move to the
next symbol.

This commit is to split the above task into 2.
- Changes to llvm-objdump and MC-layer without breaking WebAssembly code
  [ this commit ]
- AMDGPU's implementation of onSymbolStart that decodes kernel
  descriptors. [ https://reviews.llvm.org/D80713 ]

Reviewers: scott.linder, t-tye, sunfish, arsenm, jhenderson, MaskRay, aardappel

Reviewed By: scott.linder, jhenderson, aardappel

Subscribers: bcain, dschuff, wdng, tpr, sbc100, jgravelle-google, hiraditya, aheejin, MaskRay, rupprecht, llvm-commits

Tags: #llvm

Differential Revision: https://reviews.llvm.org/D80512