configure: unify ppc64 and ppc64le

configure: unify ppc64 and ppc64le

The only difference between the two, as far as either configure or
Meson are concerned, is the default endianness of the compiler.

For tests/tcg, specify the endianness explicitly on the command line;
for configure, do the same so that it is possible to have --cpu=ppc64le
on a bigendian system or vice versa.  Apart from this, cpu=ppc64le can
be normalized to ppc64 also in configure and not just in the meson
cross file.

Reviewed-by: Philippe Mathieu-Daudé <philmd@redhat.com>
Reviewed-by: Richard Henderson <richard.henderson@linaro.org>
Signed-off-by: Paolo Bonzini <pbonzini@redhat.com>