Add rustdoc html crate info
auto merge of #12918 : sfackler/rust/doc-html-attr, r=alexcrichton
doc: Remove reference to the 'extra' library

Forgot to remove this as part of the previous removal of libextra
libstd: Fix a typo. s/target_os/target_arch/
auto merge of #12921 : alexcrichton/rust/no-extra, r=sfackler

Forgot to remove this as part of the previous removal of libextra
auto merge of #12922 : luqmana/rust/fix-arm, r=alexcrichton