boards/arc/qemu_arc: Disable use of branch delay slots

I spent several hours debugging a weird stack pointer corruption bug and
discovered that QEMU appears to mess up register contents when an interrupt
fires during the execution of a branch with a delay slot.

Instead of trying to fix qemu, let's just tell the compiler to not generate
code that uses the branch instructions with delay slots.

Closes: #60071

Signed-off-by: Keith Packard <keithp@keithp.com>