[llvm-objdump] Add -M {att,intel} & deprecate --x86-asm-syntax={att,intel}

The internal `cl::opt` option --x86-asm-syntax sets the AsmParser and AsmWriter
dialect. The option is used by llc and llvm-mc tests to set the AsmWriter dialect.

This patch adds -M {att,intel} as GNU objdump compatible aliases (PR43413).

Note: the dialect is initialized when the MCAsmInfo is constructed.
`MCInstPrinter::applyTargetSpecificCLOption` is called too late and its MCAsmInfo
reference is const, so changing the `cl::opt` in
`MCInstPrinter::applyTargetSpecificCLOption` is not an option, at least without
large amount of refactoring.

Reviewed By: hoy, jhenderson, thakis

Differential Revision: https://reviews.llvm.org/D101695