powerpc: Fix bug caused by negation of 64-bit reloc_offset value

Although both ppc32 and ppc64 have a reloc_offset function, the ppc64
one produced the opposite sign to the ppc32 one.  This standardizes
on the ppc32 sign and fixes the merged 64-bit code to account for that.

Signed-off-by: Paul Mackerras <paulus@samba.org>