[gn] Rebase paths in symlink_or_copy against root_build_dir

We should be always rebasing paths against root_build_dir which is
the directory where scripts are run from, not root_out_dir which is
the current toolchain directory. The latter can result in invalid
paths when the action is being used from a non-default toolchain.

Differential Revision: https://reviews.llvm.org/D60330

llvm-svn: 357798