new port: benchmarks/flamegraph

A trivial port of Brendan Gregg's collection of flamegraph scripts (a lot of shebang-fixes). This will allow developers to visually profile output from, e.g., ljp and dtrace to locate CPU and memory hogs in workloads.

Reviewed by: swills (mentor)
Approved by: swills (mentor)
Differential Revision: https://reviews.freebsd.org/D10233