[Libomp] Place generated OpenMP headers into build resource directory (#88007)

Summary:
These headers are a part of the compiler's resource directory once
installed. However, they are currently placed in the binary directory
temporarily. This makes it more difficult to use the compiler out of the
build directory and will cause issues when moving to `liboffload`. This
patch changes the logic to write these instead to the copmiler's
resource directory inside of the build tree.

NOTE: This doesn't change the Fortran headers, I don't know enough about
those and it won't use the same directory.