runtime: make profstackdepth a GODEBUG option

runtime: make profstackdepth a GODEBUG option

Allow users to decrease the profiling stack depth back to 32 in case
they experience any problems with the new default of 128.

Users may also use this option to increase the depth up to 1024.

Change-Id: Ieaab2513024915a223239278dd97a6e161dde1cf
Reviewed-on: https://go-review.googlesource.com/c/go/+/581917
Reviewed-by: Austin Clements <austin@google.com>
LUCI-TryBot-Result: Go LUCI <golang-scoped@luci-project-accounts.iam.gserviceaccount.com>
Reviewed-by: Michael Knyszek <mknyszek@google.com>