elfcore-compat fix uid/gid types

I overlooked the difference between __kernel_uid_t and uid_t when defining
struct compat_elf_prpsinfo.  The result is a regression in 32-bit core
dumps on x86_64, where the NT_PRPSINFO note has the wrong size and layout.
This patch fixes it.

Signed-off-by: Roland McGrath <roland@redhat.com>
Acked-by: Ingo Molnar <mingo@elte.hu>
Signed-off-by: Linus Torvalds <torvalds@linux-foundation.org>