Import the NetBSD Kernel Concurrency Sanitizer (KCSAN) runtime.

KCSAN is a tool to find concurrent memory access that may race each other.
After a determined number of memory accesses a cell is created, this
describes the current access. It will then delay for a short period
to allow other CPUs a chance to race. If another CPU performs a memory
access to an overlapping region during this delay the race is reported.

This is a straight import of the NetBSD code, it will be adapted to
FreeBSD in a future commit.

Sponsored by: DARPA, AFRL