powerpc/e500mc: Set assembler machine type to e500mc

In binutils 2.26 a new opcode for the "wait" instruction was added for the
POWER9 and has precedence over the one specific to the e500mc. Commit
ebf714ff ("powerpc/e500mc: Add support for the wait instruction in
e500_idle") uses this instruction specifically on the e500mc to work around
an erratum.

This results in an invalid instruction in idle_e500 when we build for the
e500mc on bintutils >= 2.26 with the default assembler machine type.

Since multiplatform between e500 and non-e500 is not supported, set the
assembler machine type globaly when CONFIG_PPC_E500MC=y.

Signed-off-by: Michael Jeanson <mjeanson@efficios.com>
Reviewed-by: Mathieu Desnoyers <mathieu.desnoyers@efficios.com>
CC: Benjamin Herrenschmidt <benh@kernel.crashing.org>
CC: Paul Mackerras <paulus@samba.org>
CC: Michael Ellerman <mpe@ellerman.id.au>
CC: Kumar Gala <galak@kernel.crashing.org>
CC: Vakul Garg <vakul.garg@nxp.com>
CC: Scott Wood <swood@redhat.com>
CC: Mathieu Desnoyers <mathieu.desnoyers@efficios.com>
CC: linuxppc-dev@lists.ozlabs.org
CC: linux-kernel@vger.kernel.org
CC: stable@vger.kernel.org
Signed-off-by: Michael Ellerman <mpe@ellerman.id.au>