Remove gcc and zlib dependency on bootstrap-tools

The 82797f98f2917e164db8691b516ed7f354b40ff4 merge caused this
issue while trying to merge the staging refactorings with
the modular-stdenv feature branch.
Cosmetic renaming: extraPath -> extraBuildInputs
Fix zlib handling in stdenvLinux

Previously stdenv depended on two different zlibs and there was a third
one in the top-level package set for other purposes.  This commit merges
all this zlibs to one.

Actually this have been committed once as 1f2b636, but then got lost
while resolving merge conflicts.  Hopefully it survives this time.
Fix xz to not depend on bootstrap-tools
Do allowed requisites check in stdenv/linux

Use the new allowedRequisites feature in stdenvLinux.

This way we properly check that the end-result stdenv of the quite
complicated multi-stage stdenvLinux building procedure is sane, and only
depends on the stuff that we know about.

Alternative would be to just disallowRequisites bootstrapTools, which is
the most common offender, but we have had other offenders in the past.

For these checks to actually fire, you currently have to use nixUnstable,
as the necessary feature will be released in Nix 1.8.
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.
Cleanup perl logic in stdenvLinux

Make thread disabling explicit.  This changes the semantics of the perl
derivation, so on other platforms it may require setting

  enableThreading = false

This commit doesn't change the derivation or out hash of stdenvLinux.
Cleanup legacy settings in stdenvLinux

This causes a mass rebuild, because it changes the hashes of
stdenvLinux, as it enable tests for coreutils.
Fix __DATE__ and __TIME__ purification in gcc-wrapper
Disable perl 5.16 tests

They are on the path to stdenv rebuild.

Accidentally re-enabled by conflict resolution in cherry-pick 3196d4f.