Fixes #56106

spidermonkey doesn’t use the autotools build, host, target convention.
Instead it considers ‘--host’ to be the autotools’ ‘--build’ and
‘--target’ to be the autotools’ ‘--host’! As a result, we cannot
safely use “configurePlatforms”. Instead, we must manually set these
flags.

/cc @illegalprime

This avoids part of the issue where things like armv7a don’t work
because the system doesn’t realize it can use the armv7l bootstrap
tools.

Squashed to fix shell quoting, thanks @Ericson2314

ppc64le and ppc64 are different targets in the configure script. We
can’t use the same one.

TODO: canonicalize similar ones based on qemu’s configure script.

This makes us less reliant on the systems/examples.nix. You should be
able to cross compile with just your triple:

$ nix build --arg crossSystem '{ config = "armv6l-unknown-linux-gnueabi"; }' stdenv

it’s almost always a better idea to use getBin instead of .bin.
Otherwise, we could get an evaluation error if utillinux is missing
the bin otuput.

Miscellaneous cross compilation fixes