[gn][sdk] Use toolchain readelf

If `fuchsia_sdk_readelf_exec` is unset it defaults to a PATH search
and was using `/usr/bin/readelf` from a host `gcc` install.

Bug: 127321
Change-Id: I7e0cf70aacadd5779358a941dc166a107d1bceb3
Reviewed-on: https://fuchsia-review.googlesource.com/c/fuchsia/+/855839
Commit-Queue: Nathan Mulcahey <nmulcahey@google.com>
Reviewed-by: Marc-Antoine Ruel <maruel@google.com>
(cherry picked from commit 2a1f42dbd081695e846f287f715053784d7e9013)