gcc-plugins: arm_ssp_per_task_plugin: Fix for older GCC < 6

Use gen_rtx_set instead of gen_rtx_SET. The former is a wrapper macro
that handles the difference between GCC versions implementing
the latter.

This fixes the following error on my system with g++ 5.4.0 as the host
compiler

   HOSTCXX -fPIC scripts/gcc-plugins/arm_ssp_per_task_plugin.o
 scripts/gcc-plugins/arm_ssp_per_task_plugin.c:42:14: error: macro "gen_rtx_SET" requires 3 arguments, but only 2 given
          mask)),
               ^
 scripts/gcc-plugins/arm_ssp_per_task_plugin.c: In function ‘unsigned int arm_pertask_ssp_rtl_execute()’:
 scripts/gcc-plugins/arm_ssp_per_task_plugin.c:39:20: error: ‘gen_rtx_SET’ was not declared in this scope
    emit_insn_before(gen_rtx_SET
Signed-off-by: Chris Packham <chris.packham@alliedtelesis.co.nz>
Fixes: 189af465 ("ARM: smp: add support for per-task stack canaries")
Cc: stable@vger.kernel.org
Tested-by: Douglas Anderson <dianders@chromium.org>
Signed-off-by: Kees Cook <keescook@chromium.org>

scripts/gcc-plugins/arm_ssp_per_task_plugin.c

@@ -36,7 +36,7 @@ static unsigned int arm_pertask_ssp_rtl_execute(void)
 		mask = GEN_INT(sext_hwi(sp_mask, GET_MODE_PRECISION(Pmode)));
 		masked_sp = gen_reg_rtx(Pmode);
 
-		emit_insn_before(gen_rtx_SET(masked_sp,
+		emit_insn_before(gen_rtx_set(masked_sp,
 					     gen_rtx_AND(Pmode,
 							 stack_pointer_rtx,
 							 mask)),