riscv: Rewrite hartid_to_cpu in assembly

Some SBI HSM tests run without a stack being setup so they can't
run C code. Those tests still need to know the corresponding cpuid
for the hartid on which they are running. Give those tests
hartid_to_cpu() by reimplementing it in assembly.

Signed-off-by: James Raphael Tiovalen <jamestiotio@gmail.com>
Signed-off-by: Andrew Jones <andrew.jones@linux.dev>