[llvm-lib] Use COFF archive format in llvm-lib (other archive tools don't use this format).

We currently just use GNU format for llvm-lib. This mostly works, but
ARM64EC needs an additional section that does not really fit GNU format.
This patch implements writing in COFF format (as in, it's what archive
reader considers as K_COFF). This mostly requires symbol emitting symbol
map. Note that, just like in case of MSVC, symbols are de-duplicated in
both usual symbol table and the new symbol map.

Reviewed By: efriedma

Differential Revision: https://reviews.llvm.org/D143540