Add support for NetBSD/amd64

[jmmv]

This seems to work OK: the included tests pass and I've tried profiling simple demo programs (single-threaded, multi-threaded, and async using tokio) and the flame graphs look reasonable for all of them.

However, for a more complex application (the one I really want to profile), all I get in the flame graph is this:

all
124947466617856
___lwp_park60
<pprof::backtrace::backtrace_rs::Trace as pprof::backtrace::Trace>::trace
backtrace::backtrace::trace_unsynchronized
backtrace::backtrace::libunwind::trace

whereas running the same code on Linux shows a reasonable flame graph. So, something still looks amiss but I don't know what it is.

[ti-chi-bot]

Welcome @jmmv! It looks like this is your first PR to tikv/pprof-rs 🎉

[purelind]

/retest