[OpenMP] Update %preload-tool definition on Darwin

This was updated in some earlier commits but was never updated on Darwin
because I was testing locally on Linux and it does not seem like there
are any buildbots testing this configuration. Update it since it should
be trivial and will definitely be broken otherwise.

Author: boomanaiden154

openmp/runtime/test/lit.cfg

@@ -217,7 +217,7 @@ if config.has_ompt:
         config.substitutions.append(("%preload-tool", "true ||"))
         config.substitutions.append(("%no-as-needed-flag", "-Wl,--no-as-needed"))
     elif config.operating_system == 'Darwin':
-        config.substitutions.append(("%preload-tool", "env DYLD_INSERT_LIBRARIES=%T/tool.so"))
+        config.substitutions.append(("%preload-tool", "env DYLD_INSERT_LIBRARIES=%t.tool_dir/tool.so"))
         # No such linker flag on Darwin.
         config.substitutions.append(("%no-as-needed-flag", ""))
     else: