Modify bootstrapping stdenvs to make sure `targetPlatform` also passed.

This is first step towards getting rid of gcc-wrapper-cross

We now (on cross) require per-target flag interposition by putting the
triple in the names of the relevant environment variables, e.g:

export NIX_arm_unknown_linux_gnu_CFLAGS_COMPILE=...

The wrapper also has a `infixSalt` attribute (and "_" prefixed and
suffixed variants) to assist downstream packages.

Note how that the dashes are replaced to keep the identifier valid.
Using names like this allows us to keep the settings for different
compilers seperate.

I think it might be even better to use names like `NIX_{BUILD,HOST}...`
using the platform's role rather than the platform itself, but this
would be more work as the previous stages' tools would have to be re-
wrapped to take on their new role. I therefore didn't do this for now,
but that route should be thoroughly explored in the future.

Eventually we should avoid this "pre-wrapping" and just update those
files in nixpkgs. This mass-rebuild change is best done along with
those needed to reduce the disparity between native and cross (i.e.
making native the "identity cross").

…per`

…extra buildDepends

It now has the correct wrapped tools and nothing else is needed.

Packages get --host and --target by default, but can explicitly request
any subset to be passed as needed. See docs for more info.

rustc: Avoid hash breakage by using the old (ignored)
dontSetConfigureCross when not cross building

Before gcc-cross-wrapper did this unconditionally

…vars to their cross building versions

I think it's ok to export things which aren't wrapped. The cc-wrapper
can be thought of as responsible for all of binutils and the c
compiler, only wrapping those binaries which are necessary to
interposition---as opposed to all binaries it thinks are relevaant.

Conversely, adding the setup hook to the unwrapped compilers would be
unforunate as hooks are ugly hacks and the compilers themselves take
a long time to rebuild. Better to wholely separate "pure packages" from
hacks.

This is a hack until stdenv's setup.hs gets overhauled

At the next break, we can deduplicate and go for the best of both
worlds.

Without this, a `#include <float.h>` resolves incorrectly. Either the
headers weren't on the include path at all, or they only were for
local, not system, imports.

What's weird is this used to not be a problem. Not sure what other
change in e.g. cc-wrapper would affect this.

Bunch of junk for linux and cross compilation that's no longer needed

but leave the preConfigure script as it was when not cross building in order to avoid hash breakage

"--host=..." needs to be passed but crossAttrs overrides that.

check phase is now only disabled by *default* for cross, but mpfr
was forcing it always.

Check phase is now only disabled *by default* for cross, but this
package was forcing it unconditionally.

Check phase is now only disabled *by default* for cross, but this
package was forcing it unconditionally.

Carefully do not add attributes for cross unless in cross, to avoid hash breakage

Needed C toolchain targeting build platform

Do not even think about configureFlags unless in cross, to avoid hash
breaking when not in cross.

…sure to have a build cc-wrapper around as well

Get rid of gcc-cross-wrapper