[FPEnv][SystemZ] Correct strictfp tests.

Correct a SystemZ strictfp test to follow the rules documented in the LangRef:
https://llvm.org/docs/LangRef.html#constrained-floating-point-intrinsics

This test, like many others, just needed the function definition corrected.

Test changes verified with D146845.

llvm/test/CodeGen/SystemZ/sext-zext.ll

@@ -30,7 +30,7 @@ define i32 @sext_of_not_cmp(i32 %x) {
 
 ;; fold (sext (not (setcc a, b, cc))) -> (sext (setcc a, b, !cc))
 ;; make sure we don't crash if the not gets replaced in-visit
-define i32 @sext_of_not_fsetccs(double %x) {
+define i32 @sext_of_not_fsetccs(double %x) strictfp {
 ; CHECK-LABEL: sext_of_not_fsetccs:
 ; CHECK:       # %bb.0:
 ; CHECK-NEXT:    ltdbr %f0, %f0