This patch changes the top-level directory layout as a preparatory
step for improving the tools for managing 3rd-party source codes.
The rationale is described in the issue referenced below.
Issue #1082
This patch introduces a new platform 'linux_arm' for building and running
Genode/Linux on an ARM device.
Known limitations:
- libc 'setjmp()'/'longjmp()' doesn't currently save/restore floating
point registers
Fixes#746.