[llvm-objdump][AMDGPU] Detect CPU string

AMDGPU ISA isn't backwards compatible and hence -mcpu must always be specified during disassembly.
However, the AMDGPU target CPU is stored in e_flags in the ELF object.

This patch allows targets to implement CPU string detection, and also implements it for AMDGPU by looking at e_flags.

Reviewed By: scott.linder

Differential Revision: https://reviews.llvm.org/D84519