[llvm-ar] Use COFF archive format for COFF targets. (#82898)

Detect COFF files by default and allow specifying it with --format
argument.

This is important for ARM64EC, which uses a separated symbol map for EC
symbols. Since K_COFF is mostly compatible with K_GNU, this shouldn't
really make a difference for other targets.

This originally landed as #82642, but was reverted due to test failures
in tests using no symbol table. Since COFF symbol can't express it,
fallback to GNU format in that case.