[imcc] add imcc_debug flags to Parrot_debug_flags, docs

revamp --imcc-debug, improve --help-debug
remove gcdebug runcore references in the docs
remove run-exec references in harness
leave out the imcc PBC compiler options (unused)

TODO: yydebug == PARROT_IMCC_PARSER
[cage] config/gen/core_pmcs.pm: ro pm, align comments

minor config/gen/core_pmcs.pm improvements:
  remove hard tabs in generated .pm
  set buffer-read-only:t in generated .pm
  rename internal code() to c_coda(), not used for .pm
  align numbers in comments in include/parrot/core_pmcs.h