[OPENMP] fix build failure with GCC4.8

[kkwli0]

Summary

The change in kmp-os.h in https://reviews.llvm.org/D56397 causes build failure with GCC4.8 as the build compiler. This patch is to introduce a guard to get it compiled.

/gsa/tlbgsa/home/k/l/kli/llvm-project/openmp/runtime/src/kmp_os.h:303:49: error: missing binary operator before token "("
 #if __cplusplus > 201402L && __has_cpp_attribute(fallthrough)
                                                 ^
/gsa/tlbgsa/home/k/l/kli/llvm-project/openmp/runtime/src/kmp_os.h:305:26: error: missing binary operator before token "("
 #elif __has_cpp_attribute(clang::fallthrough)
                          ^
/gsa/tlbgsa/home/k/l/kli/llvm-project/openmp/runtime/src/kmp_os.h:307:22: error: missing binary operator before token "("
 #elif __has_attribute(fallthough) || _GNUC_VER >= 700
                      ^

[Hahnfeld]

As said in D57851, this will not handle Clang and only work for GCC.

openmp/runtime/src/kmp_os.h

308

This change seems right, _GNUC_VER is not defined here!

[kkwli0]

Will consolidate the review and patch in https://reviews.llvm.org/D57851 .

openmp/runtime/src/kmp_os.h

 #endif /* CACHE_LINE */
 
 #define KMP_CACHE_PREFETCH(ADDR) /* nothing */
 
 // Define attribute that indicates that the fall through from the previous 
 // case label is intentional and should not be diagnosed by a compiler
 //   Code from libcxx/include/__config
 // Use a function like macro to imply that it must be followed by a semicolon
-#if __cplusplus > 201402L && __has_cpp_attribute(fallthrough)
-#  define KMP_FALLTHROUGH() [[fallthrough]]
-#elif __has_cpp_attribute(clang::fallthrough)
-#  define KMP_FALLTHROUGH() [[clang::fallthrough]]
-#elif __has_attribute(fallthough) || _GNUC_VER >= 700
-#  define KMP_FALLTHROUGH() __attribute__((__fallthrough__))
-#else
-#  define KMP_FALLTHROUGH() ((void)0)
-#endif
+#if __GNUC__ >= 5
+#  if __cplusplus > 201402L && __has_cpp_attribute(fallthrough)
+#    define KMP_FALLTHROUGH() [[fallthrough]]
+#  elif __has_cpp_attribute(clang::fallthrough)
+#    define KMP_FALLTHROUGH() [[clang::fallthrough]]
+#  elif __has_attribute(fallthough) || __GNUC__ >= 7
+#    define KMP_FALLTHROUGH() __attribute__((__fallthrough__))
+#  else
+#    define KMP_FALLTHROUGH() ((void)0)
+#  endif
+#else
+#  define KMP_FALLTHROUGH() ((void)0)
+#endif
 
 // Define attribute that indicates a function does not return
 #if __cplusplus >= 201103L
 #define KMP_NORETURN [[noreturn]]
 #elif KMP_OS_WINDOWS
 #define KMP_NORETURN __declspec(noreturn)
 #else
 #define KMP_NORETURN __attribute__((noreturn))