doc: Remove explicit support for mips64el-linux.

doc: Remove explicit support for mips64el-linux.

It's been a good run, but no one is maintaining the architecture.
So long, and thanks for all the fish.

* doc/guix.texi (GNU Distribution): Change text for mips64el-linux
to denote it is deprecated.
(Daemon Offload Setup): Change occurrences of mips64el-linux to
aarch64-linux and adjust local code snippets.
(Guix Environment)[cross-compilation]: Change mips64el-linux-gnu to
aarch64-linux-gnu.
(GNU Build System)(package-cross-derivation]: Same.
(G-Expressions)[cross compilation]: Same.
(Additional Build Options)[cross-compilation, build logs]: Same.
(qemu-binfmt-service-type): Remove mips64el.
* doc/contributing.texi (Submitting Patches): Same.
* m4/guix.m4: (GUIX_ASSERT_SUPPORTED_SYSTEM): Remove mips64el-linux.