arm/arm64: Zero secondary CPUs' stack

For the boot CPU, the entire stack is zeroed in the entry code. For the
secondaries, only struct thread_info, which lives at the bottom of the
stack, is zeroed in thread_info_init().

Be consistent and zero the entire stack for the secondaries. This should
also improve reproducibility of the testsuite, as all the stacks now start
with the same contents, which is zero. And now that all the stacks are
zeroed in the entry code, there is no need to explicitely zero struct
thread_info in thread_info_init().

Signed-off-by: Alexandru Elisei <alexandru.elisei@arm.com>