This makes the built toolchain usable outside of MSYS2 too.
For running tests, this has another very subtle effect.
If the toolchain is built in the MSYS2 clang64 environment, the
compiler binary depends on libc++.dll. This DLL is found in $PATH,
in /clang64/bin.
When building/running tests, the current working directory will be
e.g. llvm-mingw/test/x86_64. At some point while building/running
tests, the test makefile will copy the llvm-mingw toolchain's
runtime DLLs into this directory in order to be able to execute those
newly built binaries. This also includes another, different libc++.dll.
When a libc++.dll exists in the current directory, this is preferred
over the one found further back in $PATH in /clang64/bin. As this
file is copied in parallel with other build jobs that might be
doing compilation, those compilation processes may end up loading
this other DLL. If fully copied it probably shouldn't be an issue,
but if not fully copied yet, who knows what might happen.
By making sure that there is a libc++.dll in the same directory as
the toolchain executables (rather than expecting it to be found
further back in $PATH), we ensure that this DLL is loaded and used
instead of the one in the current directory.
This should fix stray errors when running tests in the clang64
environment.
