rs6000: Allow powerpc64 to be unset for implicit 64 bit [PR108240]

rs6000: Allow powerpc64 to be unset for implicit 64 bit [PR108240]

Before r13-4894, if 64 bit is explicitly specified, option
powerpc64 is explicitly enabled too; while if 64 bit is
implicitly enabled and there is no explicit setting for
option powerpc64, option powerpc64 is eventually enabled
or not would rely on the default value of the used cpu.
It's initially set as the setting for 64 bit, but later if
the used cpu doesn't have powerpc64 supported by default,
it gets cleared.

To keep it consistent with before (also the relevant error/
warning messages), this patch is to allow that powerpc64
can be unset if 64 bit is enabled implicitly, and only stop
it from being unset if 64 bit is enabled explicitly.

Note that since the behaviors are different for implicit
and explicit 64 bit, I failed to construct one solid test
case since it becomes fragile once RUNTESTFLAGS specifying
-m64 explicitly.

	PR target/108240

gcc/ChangeLog:

	* config/rs6000/rs6000.cc (rs6000_option_override_internal): Allow
	implicit powerpc64 setting to be unset if 64 bit is enabled implicitly.