fix context switching, osx support, uart0 support in native port

Context switching:
Introduce some assembler (x86 only for the time being) to maneuver around switching RIOT context in the signal handler.
Wrap system calls to enable reentry after a signal and before RIOT context switching.

OSX:
Use darwin specific system calls where necessary, some tweaks to the build system, lost posix real time extension dependencies.

UART0:
Imperfect (since reading from stdin only in lpm at the moment) uart0 support.