Remove COMPILER_IBMXL macro for z/OS (#1777)

COMPILER_IBMXL identifies the Clang based IBM XL compiler (xlclang) on z/OS. This compiler is obsolete and replaced by the Open XL compiler, so the macro is no longer needed and the existing code would lead to incorrect asm syntax for Open XL.

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;

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)