[llvm] Mark CFGuard fn ptr symbol as DSO local and add tests for mingw

For mingw target, if a symbol is not marked DSO local, a `.refptr` is
generated for it. This makes CFG check calls use an extra pointer
dereference, which adds extra overhead compared to the MSVC version,
so mark the CFG guard check funciton pointer DSO local to stop it.
This should have no effect on MSVC target.

Also adapt the existing cfguard tests to run for mingw targets, so that
this change is checked.

Reviewed By: rnk

Differential Revision: https://reviews.llvm.org/D132331