meson: support meson 0.64 -Doptimization=plain

meson: support meson 0.64 -Doptimization=plain

In Meson 0.64, the optimization built-in option now accepts the "plain" value,
which will not set any optimization flags.  While QEMU does not check the
contents of the option and therefore does not suffer any ill effect
from the new value, it uses get_option to print the optimization flags
in the summary.  Clean the code up to remove duplication, and check for
-Doptimization=plain at the same time.

Reviewed-by: Marc-André Lureau <marcandre.lureau@redhat.com>
Signed-off-by: Paolo Bonzini <pbonzini@redhat.com>