[clang] [clang] [MinGW] Don't look for a GCC in path if the install base has a proper mingw sysroot (PR #76949)

[Martin Storsjö via cfe-commits]

mstorsjo wrote:

> > Although, on a second thought, it might actually still be good to adjust it in sync. If we're invoking Clang with `-m32` and deciding on whether to use i386/i586/i686, and we end up using the install base as sysroot, without inferring any triple from there, we shouldn't go on and check another unrelated GCC in path in order to influence this. Therefore, I think we perhaps should amend this with the following:
> 
> Yes, I think that it would be better to avoid any decisions based on an unrelated GCC and the additional check looks good to me.

Ok, updated the PR with the patch that way; will merge it sometime later if nobody has any further objections to this.

https://github.com/llvm/llvm-project/pull/76949