Currently the value is hardcoded to contain only the sysroot for
the respective darwin platform. This means that it can not be changed
in a custom toolchain file.

Instead of overriding the value, simply append it. This is similar
to how it is done in the Google provided Android toolchain file.

The usecase is to allow specifying addiitonal roots to look for
3rd party packages which are definitely not present in the default
sysroot.