[lld] [LLD] [MinGW] Fall back to using default target if no -m flag given. (PR #134700)

[Martin Storsjö via llvm-commits]

mstorsjo wrote:

> @mstorsjo I don't know if I was supposed to ping you on this. Sorry for the noise if not.

No problem, I don't mind getting CC:d. (If you add the `lld:COFF` tag on an issue or PR, it should notify a group of people including me, so that usually works as well, but I don't mind getting mentioned just to be sure.)

https://github.com/llvm/llvm-project/pull/134700