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
mk: Clean up conditions for including dist.mk
mk: Add prepare.mk

This is a slightly more generic rewrite of install.mk. Currently
used for nothing, but we'll base all the binary distributables off it.
mk: Base the install target on prepare.mk
mk: Fix a dist bug

Fallout from moving doc/ to src/doc
mk: Base the windows dist target on prepare.mk