MMIX: Replace format for private symbol output by output-time adjustment

All this started with belated MMIX regression patrol in observance of
the holidays, looking at gcc.dg/Wstringop-overflow-27.c as a
regression for target mmix.  That's because of a single message not
matched, where there is "note: destination object 'vla::22'" instead
of the expected "note: destination object 'vla'" due to
r11-5523-geafe8ee7af13c3 in which the message format and the match
changed.

That ::22 is because some identifiers that are SSA_NAME-versions and
other clones are "privatized" by ASM_FORMAT_PRIVATE_NAME and its
companion macro by default, ASM_PN_FORMAT; see the patch.  I found
that these "private names" were "unprivatized" for the purpose of
warnings and error messages *in code that only handles the default
format*, "%s.%lu".

I went ahead and wrote and tested a patch-set to hookize that
unprivatizing code, but found that it would only affect errors and
warnings; dumps still had the "target format".  While having bad
thoughts about being hit by yet another structural testism because of
the choice of outputting yet another target-specific format instead of
a canonical "versioned" format, I realized it *already was handling a
canonical format*: only the default "%s.%lu" is properly handled.

To wit, targets are better off with the default "%s.%lu" and adjusting
it (if needed, including not allowing "." or "$"), *at time of
assembly code output*.  IOW, outputs, both references and definitions,
pass a single label-output target code point: ASM_OUTPUT_LABELREF,
which is that time of output.  Some older testsuite adjustments need
to be unadjusted, but is another rabbit-hole, so I've kept that change
separate.  Other tests checking dumps, now started to pass for the
first time, some 20+.

	* config/mmix/mmix.cc (mmix_asm_output_labelref): Replace '.'
	with '::'.
	* config/mmix/mmix.h (ASM_PN_FORMAT): Define to actual default.