builtin: create builtins.c properly on old cpp

Encontered this on ancient gcc-2.95.3 environment;
src/builtins.def.in -> src/builtins.def generation emitted
^ $
lines (likely by /* */), which where NOT ignored by
src/mkbuiltins and generating bogus builtins.c.

Signed-off-by: Herbert Xu <herbert@gondor.apana.org.au>