[llvm-gsymutil] Add --symtab-file option to specify separate symbol table file  (#184059)

Add a `--symtab-file` option that allows specifying a separate file from
which
to read the symbol table during GSYM conversion. This is useful when
DWARF and
  function symbols are stored in separate files.

  Example:
`llvm-gsymutil --convert debug_info.elf --symtab-file=symbols.elf -o
output.gsym`

  ## Changes
  - Added `--symtab-file` to `Opts.td` and command-line help.
- Parsed and used `--symtab-file` in `llvm-gsymutil.cpp` during
conversion.
  - Kept architecture validation for thin object inputs.
- Added support for universal Mach-O inputs by selecting the matching
symbol-table slice for each selected
  architecture.
- Reject the case where a multi-arch universal input is converted with a
thin `--symtab-file`; `--arch` can be used
  to narrow the conversion to one architecture.
- Added lit tests covering basic ELF usage, architecture mismatch,
invalid symtab file, and thin/fat Mach-O cases.

  ## Test Plan
  - `ninja -C /tmp/llvm-gsymutil-build llvm-gsymutil`
- `/tmp/llvm-gsymutil-build/bin/llvm-lit -sv
llvm/test/tools/llvm-gsymutil/X86/elf-symtab-file.yaml llvm/test/tools/
  llvm-gsymutil/ARM_AArch64/fat-macho-symtab-file.yaml`

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Co-authored-by: Chen Li <chenlii@fb.com>