arch: riscv: pmp: fix IRQ handling of PMP stack guard

This commit add 2 minor fixes of IRQ handling:

1. Save caller registers before calling z_riscv_configure_stack_guard()
in RISC-V assembly.

2. reschedule and no_reschdule code paths use different interrupt
return path after supporting of CONFIG_PMP_STACK_GUARD. back-to-back
interrupt checking is in the reschedule code path so that it should
jump to interrupt return path of reschedule.

Signed-off-by: Jim Shu <cwshu09@gmail.com>