[Libomp] Do not error on undefined version script symbols

Summary:
A recent patch made all of the places we used version scripts in OpenMP
start throwing errors when linking with LLD. This is hopefully the last
one required to get the build bots to pass. This patch simply adds the
`--undefined-version` flag to the places where the version scripts are
used.

openmp/runtime/cmake/LibompHandleFlags.cmake

@@ -99,7 +99,7 @@ function(libomp_get_ldflags ldflags)
     IF_DEFINED CMAKE_C_OSX_COMPATIBILITY_VERSION_FLAG)
   libomp_append(ldflags_local -Wl,--warn-shared-textrel LIBOMP_HAVE_WARN_SHARED_TEXTREL_FLAG)
   libomp_append(ldflags_local -Wl,--as-needed LIBOMP_HAVE_AS_NEEDED_FLAG)
-  libomp_append(ldflags_local "-Wl,--version-script=${LIBOMP_SRC_DIR}/exports_so.txt" LIBOMP_HAVE_VERSION_SCRIPT_FLAG)
+  libomp_append(ldflags_local "-Wl,--version-script=${LIBOMP_SRC_DIR}/exports_so.txt -Wl,--undefined-version" LIBOMP_HAVE_VERSION_SCRIPT_FLAG)
   libomp_append(ldflags_local -static-libgcc LIBOMP_HAVE_STATIC_LIBGCC_FLAG)
   libomp_append(ldflags_local -Wl,-z,noexecstack LIBOMP_HAVE_Z_NOEXECSTACK_FLAG)
   libomp_append(ldflags_local -no-intel-extensions LIBOMP_HAVE_NO_INTEL_EXTENSIONS_FLAG)

openmp/runtime/cmake/config-ix.cmake

@@ -34,7 +34,7 @@ function(libomp_check_version_symbols retval)
     }")
   set(version_script_source "VER1 { }; VER2 { } VER1;")
   file(WRITE ${CMAKE_CURRENT_BINARY_DIR}/__version_script.txt "${version_script_source}")
-  set(CMAKE_REQUIRED_FLAGS -Wl,--version-script=${CMAKE_CURRENT_BINARY_DIR}/__version_script.txt)
+  set(CMAKE_REQUIRED_FLAGS "-Wl,--version-script=${CMAKE_CURRENT_BINARY_DIR}/__version_script.txt -Wl,--undefined-version")
   check_c_source_compiles("${source_code}" ${retval})
   set(${retval} ${${retval}} PARENT_SCOPE)
   file(REMOVE ${CMAKE_CURRENT_BINARY_DIR}/__version_script.txt)
@@ -132,7 +132,7 @@ elseif(NOT APPLE)
   libomp_check_linker_flag(-Wl,-x LIBOMP_HAVE_X_FLAG)
   libomp_check_linker_flag(-Wl,--warn-shared-textrel LIBOMP_HAVE_WARN_SHARED_TEXTREL_FLAG)
   libomp_check_linker_flag(-Wl,--as-needed LIBOMP_HAVE_AS_NEEDED_FLAG)
-  libomp_check_linker_flag("-Wl,--version-script=${LIBOMP_SRC_DIR}/exports_so.txt" LIBOMP_HAVE_VERSION_SCRIPT_FLAG)
+  libomp_check_linker_flag("-Wl,--version-script=${LIBOMP_SRC_DIR}/exports_so.txt -Wl,--undefined-version" LIBOMP_HAVE_VERSION_SCRIPT_FLAG)
   libomp_check_linker_flag(-static-libgcc LIBOMP_HAVE_STATIC_LIBGCC_FLAG)
   libomp_check_linker_flag(-Wl,-z,noexecstack LIBOMP_HAVE_Z_NOEXECSTACK_FLAG)
 endif()