Add experimental L4re support

This commit introduces a UNIX-alike target for the l4re microkernel
operating system. While this system is not a UNIX system at all, it is
easiest to use the POSIX interface and hence benefit from common
definitions.

This version is a very early draft, only the basic data types have been
verified and most of the complex data types (structs and unions) have
not been ported yet.