pruning stale C language variants

[Greg Troxel <gdt%lexort.com@localhost>]

I don't want to do this in freeze, but in mk/compiler/gcc-style-args.mk,
we have

  _GCC_C_DIALECTS=        c89 c90 c99 c9x c11 c1x c17 c18 c2x

I see c9x and cx1x as redundant.  I have no idea what c18 is for.  None
of these 3 otherwise appear under mk/.  So I'd like to adjust to

  _GCC_C_DIALECTS=        c89 c90 c99 c11 c17 c2x

which is a no-op because these, if in USE_CC_FEATURES, will do nothing
now.  And similarly for the gnu variants, also not appearing otherwise.

I propose to make this change post freeze.


Also c2x should now be c23, I would think.  That requires adjusting all
such USE_CC_FEATURES and FORCE_C_STD lines as part of renaming.   I
propose to do this after freeze, if it seems not massive.