Add patch to link pthread library for llvm 15 (#32838)

var/spack/repos/builtin/packages/llvm/llvm15-thread.patch

@@ -0,0 +1,23 @@
+diff --git a/openmp/libomptarget/cmake/Modules/LibomptargetGetDependencies.cmake b/openmp/libomptarget/cmake/Modules/LibomptargetGetDependencies.cmake
+index 1f2a50667c4f..d3ff232f6bd3 100644
+--- a/openmp/libomptarget/cmake/Modules/LibomptargetGetDependencies.cmake
++++ b/openmp/libomptarget/cmake/Modules/LibomptargetGetDependencies.cmake
+@@ -280,4 +280,5 @@ if (NOT LIBOMPTARGET_CUDA_TOOLKIT_ROOT_DIR_PRESET AND
+   endif()
+ endif()
+ 
+-set(OPENMP_PTHREAD_LIB ${LLVM_PTHREAD_LIB})
++find_package(Threads REQUIRED)
++set(OPENMP_PTHREAD_LIB Threads::Threads)
+diff --git a/openmp/libomptarget/src/CMakeLists.txt b/openmp/libomptarget/src/CMakeLists.txt
+index 071ec61889a2..b782c3b07e6f 100644
+--- a/openmp/libomptarget/src/CMakeLists.txt
++++ b/openmp/libomptarget/src/CMakeLists.txt
+@@ -33,6 +33,7 @@ add_llvm_library(omptarget
+   LINK_LIBS 
+   PRIVATE
+   ${CMAKE_DL_LIBS}
++  ${OPENMP_PTHREAD_LIB}
+   "-Wl,--version-script=${CMAKE_CURRENT_SOURCE_DIR}/exports"
+   NO_INSTALL_RPATH
+ )

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

@@ -360,6 +360,7 @@ class Llvm(CMakePackage, CudaPackage):
 
     # add -lpthread to build OpenMP libraries
     patch("llvm13-14-thread.patch", when="@13:14")
+    patch("llvm15-thread.patch", when="@15")
 
     # avoid build failed with Fujitsu compiler
     patch("llvm13-fujitsu.patch", when="@13 %fj")