powerpc: Optimize copyinstr() to avoid repeatedly mapping user strings

Currently copyinstr() uses fubyte() to read each byte from userspace.
However, this means that for each byte, it calls pmap_map_user_ptr() to
map the string into memory. This is needlessly wasteful, since the
string will rarely ever cross a segment boundary. Instead, map a
segment at a time, and copy as much from that segment as possible at a
time.

Measured with the HPT pmap on powerpc64, this saves roughly 8% time on
buildkernel, and 5% on buildworld, in wallclock time.