[llvm] [llvm-objdump][RISCV] Use ISA mapping symbols for per-region disassembly (PR #193448)

[Kito Cheng via llvm-commits]

kito-cheng wrote:

Changes:
- Rebase to main for including https://github.com/llvm/llvm-project/commit/5673b0215c06e93ff688baad80820f1decd377c0
- Try to combine -mattr if possible, use mapping symbol as major source if incompatible.
- Warning if mapping symbol can't parse correctly.
- Conditionally init those variable which only used by RISC-V.

https://github.com/llvm/llvm-project/pull/193448