The gcc-ar.o, gcc-nm.o, gcc-ranlib.o and errors.o included
config.h which was a generated file. But no explicity rule
to clarify the dependency. There was potential building
failure while parallel make.
For gcc-ar.o, gcc-nm.o and gcc-ranlib.o, they were compiled from one C
source file gcc-ar.c, we add them to ALL_HOST_BACKEND_OBJS, so the
'$(ALL_HOST_OBJS) : | $(generated_files)' rule could work for these
objects.
For errors.o, it is part of gengtype, and the gengtype generator program
is special: Two versions are built. One is for the build machine, and one
is for the host. We refered what gengtype-parse.o did (which also is part
of gengtype).
