[ARC] Clear the instruction cache using syscalls.

Clear the instruction cache from `beg' to `end'.  This makes an inline
system call to SYS_cacheflush.

gcc/
2017-03-28  Claudiu Zissulescu  <claziss@synopsys.com>

	* config/arc/linux.h (CLEAR_INSN_CACHE): Define.

From-SVN: r259764