base-linux: add ARM support

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.

tool/create_builddir

@@ -20,6 +20,7 @@ usage:
 	@echo
 	@echo "  <platform>   can be:"
 	@echo "                  'linux_x86'"
+	@echo "                  'linux_arm'"
 	@echo "                  'fiasco_x86'"
 	@echo "                  'pistachio_x86'"
 	@echo "                  'okl4_x86'"