Both Clang and GCC cannot generate efficient reserve_pv_entries().

http://docs.freebsd.org/cgi/mid.cgi?552BFEB2.8040407

Re-implement it entirely in inline assembly not to let compilers do silly
spilling to memory. For non-POPCNT case, use newly added bit_count(3).

Reported by: alc
Reviewed by: alc, kib
Differential Revision: https://reviews.freebsd.org/D6541