Import of compiler-rt

[Joerg Sonnenberger <joerg%britannica.bec.de@localhost>]

Hi all,
$SUBJECT says it all -- I would like to continue the quest for a GPL
free base system by importing compiler-rt. It provides a replacement for
libgcc. It's readable code and comes with a bunch of test cases that
should provide a good coverage. I expect some work to be necessary for
optimising code on the more exotic platforms, but having one file per
function already makes working on it a lot easier. The code uses the
LLVM license.

There are two important questions for usage:

(1) Is it relevant for the kernel use of the helper functions? In that
case it should go into src/common.

(2) Do we want to keep it as separate library or include it in libc.
We already have a number of support things in libc (quad/). It would
avoid the cyclic dependency and the practical difference seems to be
low otherwise.

Joerg