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.
Move the definition of '_GNU_SOURCE' from 'import-lx_hybrid.mk' to
'import-syscall.mk' because this definition may (potentially) affect all
programs that include Linux headers installed on the host - not only
hybrid Linux/Genode programs. Fixes#52
