powerpc/vdso: Retrieve sigtramp offsets at buildtime

This is copied from arm64.

Instead of using runtime generated signal trampoline offsets,
get offsets at buildtime.

If the said trampoline doesn't exist, build will fail. So no
need to check whether the trampoline exists or not in the VDSO.

Signed-off-by: Christophe Leroy <christophe.leroy@csgroup.eu>
Signed-off-by: Michael Ellerman <mpe@ellerman.id.au>
Link: https://lore.kernel.org/r/f8bfd6812c3e3678b1cdb4d55a52f9eb022b40d3.1601197618.git.christophe.leroy@csgroup.eu