powerpc/vdso32: miscellaneous optimisations

Various optimisations by inverting branches and removing
redundant instructions.

Signed-off-by: Christophe Leroy <christophe.leroy@c-s.fr>
Signed-off-by: Michael Ellerman <mpe@ellerman.id.au>
Link: https://lore.kernel.org/r/b4e79f963845545bcce1459cd6fcfe46bdde7863.1575273217.git.christophe.leroy@c-s.fr