gnu: bootstrap: Add support for powerpc-linux.

gnu: bootstrap: Add support for powerpc-linux.

On 923bb70a1bff657125c3008f119a477e5cb57c2b
   gnu:glibc-for-bootstrap: Fix patch.

Run
    ./pre-inst-env guix build --target=powerpc-linux-gnu bootstrap-tarballs

Producing

    /gnu/store/dyj1wvayyp1ihaknkxniz1xamcf4yrhl-bootstrap-tarballs-0

With guix hash -rx /gnu/store/dyj1wvayyp1ihaknkxniz1xamcf4yrhl-bootstrap-tarballs-0

    02xx2ydj28pwv3vflqffinpq1icj09gzi9icm8j4bwc4lca9irxn

* gnu/packages/bootstrap.scm (%bootstrap-executables): Add entries for
powerpc-linux.
(%bootstrap-guile-hash, %bootstrap-coreutils&co, %bootstrap-binutils,
%bootstrap-glibc, %bootstrap-gcc): Add entry for powerpc-linux.
* gnu/packages.scm (%supported-systems): Add powerpc-linux.
(%hydra-supported-systems): Remove powerpc-linux.
* m4/guix.m4: Add powerpc-linux as a supported system.