arm64: fix LD_DEAD_CODE_DATA_ELIMINATION

Keep .entry.tramp.text to avoid the "Entry trampoline text too big"
error while linking.

Bug: 62093296
Bug: 67506682
Change-Id: Idab3216244bd2f8537bb2a5bb47e25e8588394da
Signed-off-by: Sami Tolvanen <samitolvanen@google.com>