pkgs/stdenv/linux: stageFun refactoring

The stage3.extraAttrs.glibc argument was required for this whole build
procedure to correctly work and it was very-very hard to see why (the
comment said something about gcc47, but we're using gcc48 now).

This stage3.extraAttrs.glibc goes into stage3.stdenv.glibc after some
arg passing, and in pkgs/development/compiler/gcc an (stdenv ? glibc)
boolean expression decides to override /usr/include during the GCC
build.

All of our stages are built with glibc, so this refactoring moves this
repeating specification of glibc (once for gcc wrapper and once here for
extraAttrs) to stageFun, by getting rid of wrapGCC, as we were using
that in all of the stages anyways.

Incidentally it turned out, that this stdenv.glibc inconsistency caused
some random other stuff to behave differently:
  - stage1.pkgs.perl has threading disabled,
  - stage4.pkgs.coreutils (the production coreutils) has testing disabled.

Leave this historical accidents as they are in this commit, so the scope
of this commit can stay as a refactoring only, these issues will be
fixed in separate commits.

This commit doesn't change the derivation or the output hash of
stdenvLinux.