build-system/meson: Support cross-compilation.

For cross-compilation, meson needs to be passed a
‘cross file’ with information on the architecture,
CPU type, endianness and operating system, and the
name of the cross-compiler binaries.

The new module (guix build meson-configuration) has
some utilities for writing these cross files, used
by 'make-cross-file' in a G-exp.  The values for
the cross file are generated by 'make-machine-alist'
and 'make-binaries-alist'.

'make-machine-alist' and 'make-binaries-alist' live
on the host side, such that new architectures and operating
systems can be added without causing rebuilds for old
architectures.

All operating systems and targets supported by Guix are
theoretically supported, but only aarch64-linux-gnu,
powerpc64le-linux-gnu and arm-linux-gnueabihf have been
tested.  i686-linux-gnu has also been tested with a previous
version of this patch series but required some changes
to 'cross-base.scm'.

This has been tested with:

$ ./pre-inst-env guix build glib --target=TARGET

* guix/build/meson-configuration.scm
  (write-section-header): New procedure.
  (write-assignment): New procedure.
  (write-assignments): New procedure.
* guix/build-system/meson.scm
  (target-hurd?): New predicate.
  (make-machine-alist): New procedure.
  (make-binaries-alist): New procedure.
  (make-cross-file): New procedure.
  (meson-cross-build): New procedure.
  (lower)[build-inputs]: Add standard cross packages when cross-compiling.
  Do not include regular 'inputs' when cross-compiling.
  (lower)[host-inputs]: Include 'inputs' when cross-compiling.
  (lower)[target-inputs]: Add cross packages when cross-compiling.
  (lower)[build]: Call 'meson-cross-build' instead of 'cross-build'
  when cross-compiling.
  (lower)[target]: Set it.
  (lower)[private-keywords]: Do not remove #:target when cross-compiling.

Signed-off-by: Mathieu Othacehe <othacehe@gnu.org>

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