Fix Windows-x-PPC build

A recent BFD change caused a build failure for a Windows->PPC cross:

ld.exe: ../bfd/libbfd.a(coff-rs6000.o):coff-rs6000.c:(.text+0x4571): undefined reference to `getuid'
ld.exe: ../bfd/libbfd.a(coff-rs6000.o):coff-rs6000.c:(.text+0x457e): undefined reference to `getgid'

This patch fixes the problem by moving the replacement definitions of
getuid and getgid to system.h.

bfd/ChangeLog
2020-11-11  Tom Tromey  <tromey@adacore.com>

	* archive.c (getuid, getgid): Move...
	* sysdep.h (getuid, getgid): ...here.