[POWERPC] Align the sys_call_table

Our _GLOBAL macro does a ".align 2" so the alignment is fine for 32
bit, but on 64 bit it is possible for it to end up only 4 byte aligned.
I don't know if it matters, but it can't hurt to 8 byte align it.

It also means that when we build with --emit_relocs, none of our 64 bit
relocations are to misaligned places.

Signed-off-by: Stephen Rothwell <sfr@canb.auug.org.au>
Signed-off-by: Paul Mackerras <paulus@samba.org>