libcompiler-rt: update to newest version from github repo

Signed-off-by: Reto Achermann <reto.achermann@inf.ethz.ch>

Stripped compiler-rt down to what we actually use.

Anything that's in the tree, needs to be maintained. In this instance,
compiler-rt was generating Thumb code, which we cannot easily support with
Barrelfish's user-level threads.

Signed-off-by: David Cock <david.cock@inf.ethz.ch>