Copy head (r256279) to stable/10 as part of the 10.0-RELEASE cycle.

Approved by: re (implicit)
Sponsored by: The FreeBSD Foundation

Upgrade libcompiler_rt to upstream revision 147390.

This version of libcompiler_rt adds support for __mulo[sdt]i4(), which
computes a multiply and its overflow flag. There are also a lot of
cleanup fixes to headers that don't really affect us.

Updating to this revision should make it a bit easier to contribute
changes back to the LLVM developers.

Import compiler-rt r147390.