Fix casts between 64-bit physical addresses and pointers in EFI.

Fix casts between 64-bit physical addresses and pointers in EFI.

Compiling FreeBSD/i386 with modern GCC triggers warnings for various
places that convert 64-bit EFI_ADDRs to pointers and vice versa.
- Cast pointers to uintptr_t rather than to uint64_t when assigning
  to a 64-bit integer.
- Cast 64-bit integers to uintptr_t before a cast to a pointer.

Reviewed by:	kevans
MFC after:	2 weeks
Differential Revision:	https://reviews.freebsd.org/D16586