uClibc based gcc driver to provide __linux__ builtin

Newer uClibc don't export __linux__ and ideally it needs to be done by
the gcc driver. This is already provided by common gcc/config/linux.h,
ARC port was missing the necessary glue.

2013-02-28  Vineet Gupta    <vgupta@synopsys.com>

       * config/arc/arc.h (TARGET_OS_CPP_BUILTINS): Provide default

Signed-off-by: Vineet Gupta <vgupta@synopsys.com>