arch: aarch32: improve very early debugging

Debugger plugins use the `z_sys_post_kernel` variable to detect whether
the kernel is currently running, and hence whether any threads exist. As
this is just a standard variable however, after a reset the initial
value of this variable is whatever it was before reset (true) until the
bss section is zeroed halfway through `z_arm_prep_c`. Debuggers are
therefore unable to differentiate between a normally running application
and the very first stages of the boot process.

Clearing this variable as the first action upon reset allows debuggers
to display the correct thread state after the first 3 instructions have
run.

Signed-off-by: Jordan Yates <jordan.yates@data61.csiro.au>