Power10 prefixed instructions must not cross 64-byte boundaries.
This is already handled.
However, on XCOFF, the csect must be updated to match the new
alignment.

gas/ChangeLog:
2021-06-22  Clément Chigot  <clement.chigot@atos.net>

        * config/tc-ppc.c (md_assemble): Update ppc_current_csect
        alignment when finding prefixed instructions.