powerpc/vdso: Rework VDSO32 makefile to add a prefix to object files

In order to merge vdso32 and vdso64 build in following patch, rework
Makefile is order to add -32 suffix to VDSO32 object files.

Also change sigtramp.S to sigtramp32.S as VDSO64 sigtramp.S is too
different to be squashed into VDSO32 sigtramp.S at the first place.

gen_vdso_offsets.sh also becomes gen_vdso32_offsets.sh

Signed-off-by: Christophe Leroy <christophe.leroy@csgroup.eu>
Signed-off-by: Michael Ellerman <mpe@ellerman.id.au>
Link: https://lore.kernel.org/r/0c421b704a57b228e75a891512568339c53667ad.1642782130.git.christophe.leroy@csgroup.eu