Add ppc_ieee128_ok target-supports proc

  Add a target_supports entry to check that the __ieee128 keyword
is understood by the target.
Also add a dg-requires check to the existing pr92796 testcase.

    [testsuite]
	* lib/target-supports.exp (check_effective_target_ppc_ieee128_ok): New.
	* gcc.target/powerpc/pr92796.c: Add a require-effective-target
	statement for ppc_ieee128_ok.

gcc/testsuite/ChangeLog

+2020-02-11  Will Schmidt  <will_schmidt@vnet.ibm.com>
+
+	* lib/target-supports.exp (check_effective_target_ppc_ieee128_ok): New.
+	* gcc.target/powerpc/pr92796.c: Add a require-effective-target
+	statement for ppc_ieee128_ok.
+
 2020-02-11  David Malcolm  <dmalcolm@redhat.com>
 
 	PR analyzer/93374

gcc/testsuite/gcc.target/powerpc/pr92796.c

 /* { dg-do compile } */
 /* { dg-options "-O2 -fstack-protector-strong -mcpu=power8" } */
+/* { dg-require-effective-target ppc_ieee128_ok } */
 
 typedef union
 {

gcc/testsuite/lib/target-supports.exp

@@ -2250,6 +2250,25 @@ proc check_ppc_float128_hw_available { } {
     }]
 }
 
+# See if the __ieee128 keyword is understood.
+proc check_effective_target_ppc_ieee128_ok { } {
+    return [check_cached_effective_target ppc_ieee128_ok {
+	# disable on AIX.
+	if { [istarget *-*-aix*] } {
+	    expr 0
+	} else {
+	    set options "-mfloat128"
+	    check_runtime_nocache ppc_ieee128_ok {
+		int main()
+		{
+		  __ieee128 a;
+		  return 0;
+		}
+	    } $options
+	}
+    }]
+}
+
 # Return 1 if the target supports executing VSX instructions, 0
 # otherwise.  Cache the result.