[clang] [Clang][LoongArch] Generate _mcount instead of mcount (PR #65657)

[WÁNG Xuěruì via cfe-commits]

xen0n wrote:

also cc @SixWeining (BTW we want to have a GitHub team set up for notifications and assignments apparently)

It's 4 am here and I'd take a look after getting some sleep, to ensure review quality ;-)

https://github.com/llvm/llvm-project/pull/65657