[SystemZ][z/OS] Remove COMPILER_IBMXL macro for z/OS (#87493)

This copies the change made in google benchmark
(70916cbf71)
to remove COMPILER_IBMXL for z/OS.

third-party/benchmark/src/cycleclock.h

@@ -181,10 +181,11 @@ inline BENCHMARK_ALWAYS_INLINE int64_t Now() {
 #elif defined(__s390__)  // Covers both s390 and s390x.
   // Return the CPU clock.
   uint64_t tsc;
-#if defined(BENCHMARK_OS_ZOS) && defined(COMPILER_IBMXL)
-  // z/OS XL compiler HLASM syntax.
+#if defined(BENCHMARK_OS_ZOS)
+  // z/OS HLASM syntax.
   asm(" stck %0" : "=m"(tsc) : : "cc");
 #else
+  // Linux on Z syntax.
   asm("stck %0" : "=Q"(tsc) : : "cc");
 #endif
   return tsc;

third-party/benchmark/src/internal_macros.h

@@ -11,11 +11,7 @@
 #endif
 
 #if defined(__clang__)
-  #if defined(__ibmxl__)
-    #if !defined(COMPILER_IBMXL)
-      #define COMPILER_IBMXL
-    #endif
-  #elif !defined(COMPILER_CLANG)
+  #if !defined(COMPILER_CLANG)
     #define COMPILER_CLANG
   #endif
 #elif defined(_MSC_VER)