FROMLIST: kheaders: dereferences the source tree

When the kernel is built inside a sandbox container,
a forest of symlinks to the source files may be
created in the container. In this case, the generated
kheaders.tar.xz should follow these symlinks
to access the source files, instead of packing
the symlinks themselves.

Test: manual (add kheaders_data.tar.xz to the output,
  then examine the contents)
Bug: 276339429
Fixes: b0acbba3 ("Revert "Revert "Revert "FROMLIST: kheaders: Follow symlinks to source files."""")
Link: https://lore.kernel.org/lkml/20230420010029.2702543-1-elsk@google.com/


Signed-off-by: Yifan Hong <elsk@google.com>
(cherry picked from https://android-review.googlesource.com/q/commit:28fa7afc424f3dc53358c0e9b080433d78f0cd54)
Merged-In: Ie4db22dfa13d05fdccb3ad8f4fae2fe3fead994e
Change-Id: Ie4db22dfa13d05fdccb3ad8f4fae2fe3fead994e