Update restrictions on rustdoc source rendering

The std macros used to be injected with a filename of "<std-macros>", but macros
are now injected with a filename of "<{} macros>" where `{}` is filled in with
the crate name. This updates rustdoc to understand this new system so it'll
render source more frequently.
auto merge of #12277 : alexcrichton/rust/fix-rustdoc-render, r=huonw

The std macros used to be injected with a filename of "<std-macros>", but macros
are now injected with a filename of "<{} macros>" where `{}` is filled in with
the crate name. This updates rustdoc to understand this new system so it'll
render source more frequently.
Enable 64-bit checked multiplication on 32-bit

This was just waiting for compiler-rt support, which was added in #12027

Closes #8449
auto merge of #12276 : alexcrichton/rust/issue-8449, r=kballard

This was just waiting for compiler-rt support, which was added in #12027

Closes #8449