powerpc/Makefile: Don't prefix archhelp commands with "@"

It's not necessary to prefix every command in archhelp with "@" (to
suppress echoing the command), because that is done by the top level
Makefile when it evaluates archhelp.

Signed-off-by: Michael Ellerman <mpe@ellerman.id.au>
Link: https://msgid.link/20230329072334.2023357-1-mpe@ellerman.id.au