bug#29644: gcc-objc is unusable without its 'gcc' executable

[宋文武]

Hello, unlike fortran program files which can be compiled using the
command 'gfortran' (in addition to 'gcc'), there is no other command for
Objective-C program files, and run 'gcc -c x.m' using the 'gcc' package
will just complain "Objective-C compiler not installed on this system"
due to it lacking objc support.

I have to revert commit 82f145ef7a to get a objc enabled 'gcc' in the
'gcc-objc' package.

IIUC, the purpose of that commit is to avoid file collisions between
'gfortran' and the 'gcc' package used in the 'gnu-build-system', which
will broke the compiler in some way.  So I think we really want to only
have one gcc package in an environment...

How about enable all languages (except 'brig' which I never heard of)
for the gcc-final and the 'gcc' (in gcc.scm) packages?  In this way, I
think 'gnu-build-system' and 'gcc-toolchain' will able to compile
Fortran, Objective-C, Go, etc. out of the box.