[LLVM][Tests] remove %exeext from lli-child-target parameter.

[jeremyd2019]

Its presence seems to actively hinder the ToolSubst mechanism that was supposed to fill in the path to the tool, which prevented these tests from working on Cygwin.

[jeremyd2019]

cc @mstorsjo

Given these tests are already expected to fail on Windows otherwise, this was probably just an oversight. Unless there's some other platform that uses %exeext that this might break.

This brings llvm test failures on Cygwin from 50 down to 39.

[mstorsjo]

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.

[llvm-ci]

LLVM Buildbot has detected a new failure on builder publish-sphinx-docs running on as-worker-4 while building llvm at step 12 "Publish docs-openmp-html".

Full details are available at: https://lab.llvm.org/buildbot/#/builders/45/builds/13078

Here is the relevant piece of the build log for the reference

Step 12 (Publish docs-openmp-html) failure: 'rsync -vrl ...' (failure)
kex_exchange_identification: Connection closed by remote host
rsync: connection unexpectedly closed (0 bytes received so far) [sender]
rsync error: unexplained error (code 255) at io.c(235) [sender=3.1.3]