powerpc/vdso: Fix build rules to rebuild vdsos correctly

When using if_changed, we need to add FORCE as a dependency (see
Documentation/kbuild/makefiles.txt) otherwise we don't get command line
change checking amongst other things. This has resulted in vdsos not
being rebuilt when switching between big and little endian.

The vdso64/32ld commands have to be changed around to avoid pulling
FORCE into the linker command line (code copied from x86).

Signed-off-by: Nicholas Piggin <npiggin@gmail.com>
Signed-off-by: Michael Ellerman <mpe@ellerman.id.au>