libgomp: Fix up build on mingw [PR107641]

Pointers should be first casted to intptr_t/uintptr_t before casting
them to another integral type to avoid warnings.
Furthermore, the function has code like
  else if (upper <= UINT_MAX)
    something;
  else
    something_else;
so it seems using unsigned type for upper where upper <= UINT_MAX is always
true is not intended.

2022-11-12  Jakub Jelinek  <jakub@redhat.com>

	PR libgomp/107641
	* env.c (parse_unsigned_long): Cast params[2] to uintptr_t rather than
	unsigned long.  Change type of upper from unsigned to unsigned long.