Previously, C-- was processed with the C preprocessor program.  This
means that it inherited flags passed via -optc.  A flag that is somewhat
often passed through -optc is -g.  At certain -g levels (>=2), GCC
starts emitting defines *after* preprocessing, for the purposes of
debug info generation.  This is not useful for the C-- compiler, and, in
fact, causes lexer errors.  We can suppress this effect (safely, if
supported) via -g0.

As a workaround, in older versions of GCC (<=10), GCC only emitted
defines if a certain set of -g*3 flags was passed.  Newer versions check
the debug level.  For the former, we filter out those -g*3 flags and,
for the latter, we specify -g0 on top of that.

As a compatible and effective solution, this change adds a C--
preprocessor distinct from the C compiler and preprocessor, but that
keeps its flags.  The command line produced for C-- preprocessing now
looks like:

  $pgmCmmP $optCs_without_g3 $g0_if_supported $optCmmP

Closes: ghc/ghc#24474