ARM: decompressor: split off _edata and stack base into separate object

In preparation of moving the handling of the LC0 object to a later stage
in the decompressor startup code, move out _edata and the initial value
of the stack pointer, which are needed earlier than the remaining
contents of LC0.

Signed-off-by: Ard Biesheuvel <ardb@kernel.org>
Reviewed-by: Geert Uytterhoeven <geert+renesas@glider.be>
Reviewed-by: Nicolas Pitre <nico@fluxnic.net>