Merge #21323: guix, doc: Update default HOSTS value

a0a7a4337d guix, doc: Update default HOSTS value (Hennadii Stepanov)

Pull request description:

  This is a #21089 follow up.

ACKs for top commit:
  fanquake:
    ACK a0a7a4337d

Tree-SHA512: c1813cc2b9212a79fd34d4e25cd0816b58264e1890daf777cd59411bd20fcc9affe312871d06fab1308b8f55c1a78ac1101e631882c18360a4709ecef4529f05

contrib/guix/README.md

@@ -103,8 +103,8 @@ find output/ -type f -print0 | sort -z | xargs -r0 sha256sum
 
   Override the space-separated list of platform triples for which to perform a
   bootstrappable build. _(defaults to "x86\_64-linux-gnu arm-linux-gnueabihf
-  aarch64-linux-gnu riscv64-linux-gnu x86_64-w64-mingw32
-  x86_64-apple-darwin18")_
+  aarch64-linux-gnu riscv64-linux-gnu powerpc64-linux-gnu powerpc64le-linux-gnu
+  x86\_64-w64-mingw32 x86\_64-apple-darwin18")_
 
 * _**SOURCES_PATH**_