Add patch to fix issue building current llvm develop master on power9 (#13986)

* Add patch to fix issue building current llvm develop master on power9

* Conform to proper block commenting

Co-Authored-By: Adam J. Stewart <ajstewart426@gmail.com>

var/spack/repos/builtin/packages/llvm/package.py

@@ -203,6 +203,9 @@ class Llvm(CMakePackage):
     # for a bug report about this problem in llvm master.
     patch('llvm_py37.patch', when='@4:6 ^python@3.7:')
 
+    # https://bugs.llvm.org/show_bug.cgi?id=39696
+    patch('thread-p9.patch', when='@develop+libcxx')
+
     @run_before('cmake')
     def check_darwin_lldb_codesign_requirement(self):
         if not self.spec.satisfies('+lldb platform=darwin'):

var/spack/repos/builtin/packages/llvm/thread-p9.patch

@@ -0,0 +1,16 @@
+diff --git a/libcxx/include/thread b/libcxx/include/thread
+index 02da703..d1677a1 100644
+--- a/projects/libcxx/include/thread
++++ b/projects/libcxx/include/thread
+@@ -368,9 +368,9 @@ sleep_for(const chrono::duration<_Rep, _Period>& __d)
+     {
+ #if defined(_LIBCPP_COMPILER_GCC) && (__powerpc__ || __POWERPC__)
+     //  GCC's long double const folding is incomplete for IBM128 long doubles.
+-        _LIBCPP_CONSTEXPR duration<long double> _Max = nanoseconds::max();
+-#else
+         _LIBCPP_CONSTEXPR duration<long double> _Max = duration<long double>(ULLONG_MAX/1000000000ULL) ;
++#else
++        _LIBCPP_CONSTEXPR duration<long double> _Max = nanoseconds::max();
+ #endif
+         nanoseconds __ns;
+         if (__d < _Max)