powerpc/inst: Refactor ___get_user_instr()

PPC64 version of ___get_user_instr() can be used for PPC32 as well,
by simply disabling the suffix part with IS_ENABLED(CONFIG_PPC64).

Signed-off-by: Christophe Leroy <christophe.leroy@csgroup.eu>
Signed-off-by: Michael Ellerman <mpe@ellerman.id.au>
Link: https://lore.kernel.org/r/1f0ede830ccb33a659119a55cb590820c27004db.1638208156.git.christophe.leroy@csgroup.eu