[llvm] [Windows] Use absolute path when loading system shared library (PR #90520)

[via llvm-commits]

jofrn wrote:

> And it most probably can make binaries unportable, in case the files are available in different directories on different machines.
It depends on how portable find_library searching vs LoadProcessW searching compare to one another. find_library is more general and configurable. We can restrict its search path further; having it match LoadProcessW for all Windows might be tricky.

> This doesn't work when cross compiling, because the files used at runtime won't be available there. 
Will take a look... thank you.

https://github.com/llvm/llvm-project/pull/90520