[SystemZ] Make lit test more specific (#115050)

The lit test fmuladd-soft-float.ll only specifies s390x as platform,
but the test is Linux specific, causing problems when run on z/OS.
This change updates the triple to fix this.

Author: redstar

llvm/test/CodeGen/SystemZ/fmuladd-soft-float.ll

@@ -1,5 +1,5 @@
 ; NOTE: Assertions have been autogenerated by utils/update_llc_test_checks.py UTC_ARGS: --version 5
-; RUN: llc -mtriple=s390x < %s | FileCheck %s -check-prefix=SOFT-FLOAT
+; RUN: llc -mtriple=s390x-linux < %s | FileCheck %s -check-prefix=SOFT-FLOAT
 
 define float @fmuladd_intrinsic_f32(float %a, float %b, float %c) #0 {
 ; SOFT-FLOAT-LABEL: fmuladd_intrinsic_f32: