arm: fix early boot on Cortex-M0 with init stack

The assembler was passed immediate values that are too large for the
limited Cortex-M0 thumb assembly. Load values in registers instead of
using immediate values.

Change-Id: Ib5541c92dea03e0efb1b88ab91eeb408d151a71b
Signed-off-by: Benjamin Walsh <benjamin.walsh@windriver.com>