[gn/mac] Fix isysroot flag when building compiler-rt

If the GN arg `sysroot` was set to a root-relative path like e.g.
`"//sysroot"`, we weren't rebasing it correctly, leading to
warnings from clang about it ignoring a non-existent sysroot.

Similar to 6073f87.

Author: nico

llvm/utils/gn/build/toolchain/target_flags.gni

@@ -44,7 +44,7 @@ if (current_os == "android") {
   if (current_os == "mac") {
     target_flags += [
       "-isysroot",
-      mac_sdk_path,
+      rebase_path(mac_sdk_path, root_build_dir),
       # TODO(lgrey): We should be getting this from `compiler_defaults`. Why
       # aren't we?
     "-mmacos-version-min=10.10",