[LLD] [COFF] Order .debug_* sections at the end, to avoid leaving gaps if stripped

So far, we sort all discardable sections at the end, with only some
extra logic to make sure that the .reloc section is at the start
of that group of sections. But if there are other discardable
sections, other than .reloc, they must also be ordered before
.debug_* sections, to avoid leaving gaps if the executable is
stripped.

(Stripping executables doesn't remove all discardable sections,
only the ones named .debug_*).

Rust binaries seem to include a .rmeta section, which is marked
discardable. This fixes stripping such binaries if built with
dwarf debug info included.

This fixes issues observed in MSYS2 in
https://github.com/msys2/MINGW-packages/pull/10555.

Differential Revision: https://reviews.llvm.org/D120805