arch: riscv: stacktrace: fix cpuid type and optimize branch with compiler

Change the type of `cpu_id` to `uint8_t` since that is the type
of `arch_curr_cpu()->id`.

Instead of using precompiler switch (`#ifdef CONFIG_SMP`), use
if-else shorthand instead (`IS_ENABLED(CONFIG_SMP)`).

Signed-off-by: Yong Cong Sin <ycsin@meta.com>