kboot: Implement stat(2) and fstat(2) system calls

Implement stat(2) and fstat(2) in terms of newfstatat and newfstat
system calls respectively (assume we have a compat #define when
there's no newfstat and just a regular fstat and do so for ppc).

Snag struct kstat (the Linux kernel stat(2), et al interface) from musl
and attribute properly.

Sponsored by: Netflix

(cherry picked from commit a647d4a4d15df856d590c39c19d3973ec2e18825)