Replace the last non-optional use of sbrk() in the tree with mmap().

All gmon want's is a region of memory without the overhead of malloc().
Just mapping some pages with mmap is an easy way to accomplish this.

Approved by: jhb, cem, emaste
Obtained from: CheriBSD (bf33e1e70b368ababde74aa3ac70d108c8a52c69)
Sponsored by: DARPA, AFRL
Differential Revision: https://reviews.freebsd.org/D5005