Move (gnu platform) and (gnu platforms ...) to guix/.

Move (gnu platform) and (gnu platforms ...) to guix/.

* gnu/platform.scm:
* gnu/platforms/arm.scm:
* gnu/platforms/hurd.scm:
* gnu/platforms/mips.scm:
* gnu/platforms/powerpc.scm:
* gnu/platforms/riscv.scm:
* gnu/platforms/s390.scm:
* gnu/platforms/x86.scm: Move to guix/.

* Makefile.am:
* doc/guix.texi (Porting to a New Platform):
* etc/release-manifest.scm:
* gnu/ci.scm:
* gnu/image.scm:
* gnu/local.mk:
* gnu/packages/bioinformatics.scm:
* gnu/packages/bootstrap.scm:
* gnu/packages/cross-base.scm:
* gnu/packages/instrumentation.scm:
* gnu/packages/linux.scm:
* gnu/system/image.scm:
* gnu/system/images/hurd.scm:
* gnu/system/images/novena.scm:
* gnu/system/images/pine64.scm:
* gnu/system/images/pinebook-pro.scm:
* gnu/system/images/rock64.scm:
* guix/scripts/build.scm:
* guix/scripts/system.scm:
* guix/self.scm: Update (gnu platform...) to (guix platform...).

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