runtime: respect GOTRACEBACK for user-triggered runtime panics

The documentation for GOTRACEBACK says that "single" is the default
where the stack trace for only a single routine is printed except
that it prints all stack traces if:

	there is no current goroutine or
	the failure is internal to the run-time.

In the runtime, there are two types of panics:
throwTypeUser and throwTypeRuntime.
The latter more clearly corresponds to a
"failure [that] is internal to the run-time",
while the former corresponds to a
problem trigger due to a user mistake.

Thus, a user-triggered panic (e.g., concurrent map access)
should not result in a dump of all stack traces.

Fixes #68019

Change-Id: I9b02f82535ddb9fd666f7158e2e4ee10f235646a
Reviewed-on: https://go-review.googlesource.com/c/go/+/649535
LUCI-TryBot-Result: Go LUCI <golang-scoped@luci-project-accounts.iam.gserviceaccount.com>
Reviewed-by: Michael Pratt <mpratt@google.com>
Reviewed-by: Michael Knyszek <mknyszek@google.com>