`thread_stack_init()` didn't correctly set up the stack alignment.
This fixes the issue and brings the function closer to the Cortex M
version, laying the groundwork for future code duplication.
This fixes https://github.com/RIOT-OS/RIOT/issues/11885
- Enforced that ISR_STACKSIZE is indeed a multiple of 4
- With this enforced, every cast that triggers a -Wcast-align warning is now
a false positives, so those were silenced by (intermediately) casting to
`uintptr_t`.
This didn't change binaries for me. Either the linker script already took care
of it through the section names of the stacks, or I just was lucky. If I was
just luck, this fixes a bug. If not, it makes the hidden alignment explicit in
the C code, so that code review is easier.
- removed ISR_STACKSIZE define where unused (set to 0)
- removed thread_arch_isr_stack_usage(), thread_arch_isr_stack_start(),
and/or thread_arch_isr_stack_pointer() where not implemented
