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

[via llvm-commits]

jeremyd2019 wrote:

Compared to what I was playing with in #134558 I decided to create a Triple variable in `mingw::link` since this `isI386Target` function is now called twice.  Also, more cygwin-related hackery I'm playing with is going to have to depend on the target being `isWindowsCygwinEnvironment` so having the target there as a variable will be helpful for that too.

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