In preparation of compiling our kernels with -ffreestanding, explicitly map
a few performance-critical functions to compiler builtins. Since the
builtins supported by gcc3, gcc4 and clang are not the same, there are
(unfortunately) some compiler checks to make sure we only do the mapping
for builtins that are actually supported by the compiler.

ok jca@, tom@, guenther@

take libc bcopy, gut the #ifdefs for other modes, and place it here.
Now deficient architectures can use this if they need to.
conf/files already tries to pull this in (pointed out by mlarkin)