[jitlink/rtdydl][checker] Add TargetFlag dependent disassembler switching support

Some targets such as AArch32 make use of TargetFlags to indicate ISA mode. Depending
on the TargetFlag, MCDisassembler and similar target specific objects should be
reinitialized with the correct Target Triple. Backends with similar needs can
easily extend this implementation for their usecase.

The drivers llvm-rtdyld and llvm-jitlink have their SymbolInfo's extended to take
TargetFlag into account. RuntimeDyldChecker can now create necessary TargetInfo
to reinitialize MCDisassembler and MCInstPrinter. The required triple is obtained
from the new getTripleFromTargetFlag function by checking the TargetFlag.

In addition, breaking changes for RuntimeDyld COFF Thumb tests are fixed by making
the backend emit a TargetFlag.

Reviewed By: lhames, sgraenitz

Differential Revision: https://reviews.llvm.org/D158280