arm: add software reboot for QEMU

This simulates a reboot by jumping back to the address stored in the
reset vector in vector table found at address 0. It is supported from
interrupt/exception level, which means that sys_arch_reboot() in this
case finds out if it is called from thread mode or handler mode, and in
the latter case, it unwinds the nested exception stack as needed.

Change-Id: Ib67f850f8411f1ee8fc592a5f31f2f70d0af14a4
Signed-off-by: Benjamin Walsh <benjamin.walsh@windriver.com>