RISC-V: initialize pcpu slightly earlier

In certain scenarios, it is possible for PCPU data to be
accessed before it has been initialized (e.g. during printf
if the kernel was built with the TSLOG option).

Initialize the PCPU pointer for hart 0 at the beginning of
initriscv() rather than near the end.

Reviewed by: markj
Approved by: markj (mentor)
Differential Revision: https://reviews.freebsd.org/D19726