powerpc: Work around ld bug in older binutils

Commit 549e8152 ("powerpc: Make the
64-bit kernel as a position-independent executable") added lines to
vmlinux.lds.S to add the extra sections needed to implement a
relocatable kernel.  However, those lines seem to trigger a bug in
older versions of GNU ld (such as 2.16.1) when building a
non-relocatable kernel.  Since ld 2.16.1 is still a popular choice for
cross-toolchains, this adds an #ifdef to vmlinux.lds.S so the added
lines are only included when building a relocatable kernel.

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