[ARM] Cleanup startup code for primary and secondary CPUs

This patch cleans startup code of arm32 and arm32ve.

Now we have two distinct entry points for primary (i.e. _start) and secondary (i.e. _start_secondary) CPUs.

Also, primary CPU startup = lot of primary CPU initialization + secondary CPU startup hence, _start_secondary would be last part of _start.

Signed-off-by: Anup Patel <anup@brainfault.org>