[gn] Rebase paths in symlink_or_copy against root_build_dir
[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

git-svn-id: 0ff597fd157e6f4fc38580e8d64ab130330d2411@357798 91177308-0d34-0410-b5e6-96231b3b80d8

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