[llvm] [LLVM][Tests] remove %exeext from lli-child-target parameter. (PR #143077)

[Martin Storsjö via llvm-commits]

https://github.com/mstorsjo approved this pull request.

LGTM.

I can confirm that on a mingw target, this previously passed `-mcjit-remote-process=lli-child-target.exe`, while it now passes `-mcjit-remote-process=c:\full\path\to\build\bin\lli-child-target.exe`. The tests still fail expectedly like before.

https://github.com/llvm/llvm-project/pull/143077