Force HAVE_LIB_RT to NO when building with Xcode toolchains

[jroelofs]

Summary

Fixes an issue revealed by https://reviews.llvm.org/D121343, causing several of the test-suite-verify-machineinstrs* bots on GreenDragon to fail with ld: library not found for -lrt

rdar://91014785

[fhahn]

LGTM as workaround to get the bots back to working.

[jroelofs]

33c34454f18c5086eae797850992692e60bed11b

cmake/caches/util/xcode_sdk.cmake

DYLD_LIBRARY_PATH=\"${COMPILER_DIR}/../lib:$DYLD_LIBRARY_PATH\" ${SDK_TOOL_BIN} \"$@\"
   set(CMAKE_TRY_COMPILE_TARGET_TYPE STATIC_LIBRARY CACHE STRING "")
 
   # Some benchmarks check for the function re_comp by using
   # check_function_exists. This function is not available in any Xcode SDK, and
   # the check returns YES with custom built compilers when we cross-compile,
   # because of "CMAKE_TRY_COMPILE_TARGET_TYPE STATIC_LIBRARY" explained just
   # above. We work around that by forcing it to NO.
   set(HAVE_RE_COMP CACHE BOOL NO)
+
+  # Likewise, some benchmarks look for librt, which does not exist on darwin
+  # systems, but the check returns YES because we don't actually link, given
+  # the CMAKE_TRY_COMPILE_TARGET_TYPE setting above.  We work around that by
+  # forcing it to NO.
+  set(HAVE_LIB_RT CACHE BOOL NO)
 else()
   # Search and use compiler coming with the SDK.
   # Note that we do not search CMAKE_CXX_COMPILER here. cmake will try
   # `${CMAKE_C_COMPILER}++` and use that if available.
   xcrun_find_update_cache(CMAKE_AR ar)
   xcrun_find_update_cache(CMAKE_NM nm)
   xcrun_find_update_cache(CMAKE_C_COMPILER clang)
   xcrun_find_update_cache(CMAKE_RANLIB ranlib)