gcc-testsuite: Fix ppc cpu specification

After this change in qemu:

https://gitlab.com/qemu-project/qemu/-/commit/c7e89de13224c1e6409152602ac760ac91f606b4



there is no 'max' cpu model on ppc. Drop it to clean up ppc gcc testsuite failures.

In order for this to work we do need to pull in the alternative cpu option from
QEMU_EXTRAOPTIONS on powerpc.

Signed-off-by: Richard Purdie <richard.purdie@linuxfoundation.org>
(cherry picked from commit c447f2b2)
Signed-off-by: Steve Sakoman <steve@sakoman.com>