Import the NetBSD micro ubsan code for the kernel.

This imports revision 1.3 of common/lib/libc/misc/ubsan.c from NetBSD, the
micro-ubsan code. It is an implementation of the Undefined Behavior
Sanitizer runtime for use with recent clang and gcc.

The uubsan code will be used in a later commit to implement kubsan to help
find undefined behavior in the kernel.

Sponsored by: DARPA, AFRL