lua5.1: fix cross-compilation
Add cross-arm.nix (for cross-compiling for ARM)
cross-arm.nix: update
Add embedded.txt
glibc: remove runtime dependency on build-sys bash when cross-compiling

The sotruss tool from the glibc package has a reference to build-sys
bash; its shebang is /bin/sh and the standard builder changes that to
/nix/store/HASH-bash/bin/sh in the fixupPhase.

There is already a list of tools that are removed when cross-compiling
to avoid depending on build-sys bash. Add sotruss to that list.
kbd: remove runtime dependency on build system bash when cross-compiling
kbd: fix dependency on gzip, bzip2

When cross-compiling we want to depend on the cross-compiled bzip and
bzip2, not the ones built for the build machine.
python: fix cross-compilation

Patches and configure flags come from Buildroot.
libcap: fix cross-compilation
libxslt: fix cross-compilation
tcl: fix cross-compilation
bison: fix cross-compilation
expect: fix cross-compilation

Borrow cross-compilation patch from Buildroot.

Also, change "patchPhase" to "postPatch" so that the standard
"patchPhase" still works (i.e. "patches = [ ... ]" are applied).

BUG: Sometimes cross-compilation fails with an error simiar to when the
cross-compilation patch isn't applied(!).
WIP: perl: fix cross-compilation (FIXME: doesn't build yet; perl-cross patches don't apply)

When cross-compiling, use the perl-cross "overlay" to provide a sane
(wrt. cross-compilation) configure/build system.