enhance GCC easyblock to also put symlinks for cc/c++/f77/f95 in place

[boegel]

fix for #507

[akesandgren]

I would personally not do the chdir in and out, and instead change the symlink to:
symlink(comp_cmd_symlinks[key], target, ... instead. To avoid the basename call and still keep the symlink short-relative

[boegel]

Thanks for the suggestion @akesandgren!

Retested, works like a charm:

$ ls -l $EBROOTGCCCORE/bin | egrep ' cc| c\+\+| f77 | f95'
-rwxr-xr-x 4 vsc40023 vsc40023 4566920 Sep 20 15:36 c++
lrwxrwxrwx 1 vsc40023 vsc40023       3 Sep 20 15:36 cc -> gcc
lrwxrwxrwx 1 vsc40023 vsc40023       8 Sep 20 15:36 f77 -> gfortran
lrwxrwxrwx 1 vsc40023 vsc40023       8 Sep 20 15:36 f95 -> gfortran

[akesandgren]

old_cmds should be taken from comp_cmd_symlinks and not written twice in the code.