[OpenMP] Fix a few build issues

Fix two build issues:

1) Recent commit 348756 accidentally included Unix clang compilers
   to use immintrin.h when only clang-cl should be using it leading
   to the following error:

openmp-llvm/runtime/src/kmp_lock.cpp:2035:25: error: always_
inline function '_xbegin' requires target feature 'rtm', but would be inlined into function
      '__kmp_test_adaptive_lock_only' that is compiled without support for 'rtm'
          kmp_uint32 status = _xbegin();
This patch changes the guard to use immintrin.h to only use clang-cl instead of all clang

2) gcc-8 gives a warning about multiline comment in kmp_runtime.cpp:
This patch just changes it to a two line comment
openmp-llvm/runtime/src/kmp_runtime.cpp:7697:8: warning: multi-line comment [-Wcomment]
 #endif // KMP_OS_LINUX || KMP_OS_DRAGONFLY || KMP_OS_FREEBSD || KMP_OS_NETBSD  \

llvm-svn: 348783