powerpc/boot: Fix crt0.S current address branch form

Use the preferred form of branch-and-link for finding the current
address so objtool doesn't think it is an unannotated intra-function
call.

Signed-off-by: Nicholas Piggin <npiggin@gmail.com>
Signed-off-by: Michael Ellerman <mpe@ellerman.id.au>
Link: https://msgid.link/20230407040924.231023-1-npiggin@gmail.com