arm64: linker: leave app_smem before .data

Commit 87f68b4d ("arm64: linker: move data section between rodata
and bss") moved .data up a little too far. Keep app_smem in front of it
otherwise user memory partitions end up in general .data with no user
permissions.

Signed-off-by: Nicolas Pitre <npitre@baylibre.com>