[mlir][GPU] Initialize LLVM exactly once during GPU compiles

No matter how one constructs their SerializeTo* pass, we want to
ensure that the LLVM initialization code runs once and only once. This
commit adds a static once_flag to ensure that.

I've run into mysterious segfaults when calling MLIR GPU compiles from
multiple threads, and this commit is a potential fix for the issue.

Reviewed By: fmorac

Differential Revision: https://reviews.llvm.org/D155226