runtime: use m.profStack in traceStack

runtime: use m.profStack in traceStack

Turns out we spend a few percent of the trace event writing path in just
zero-initializing the stack space for pcBuf. We don't need zero
initialization, since we're going to write over whatever we actually
use. Use m.profStack instead, which is already sized correctly.

A side-effect of this change is that trace stacks now obey the GODEBUG
profstackdepth where they previously ignored it. The name clearly
doesn't match, but this is a positive: there's no reason the maximum
stack depth shouldn't apply to every diagnostic.

Change-Id: Ia654d3d708f15cbb2e1d95af196ae10b07a65df2
Reviewed-on: https://go-review.googlesource.com/c/go/+/723062
LUCI-TryBot-Result: Go LUCI <golang-scoped@luci-project-accounts.iam.gserviceaccount.com>
Reviewed-by: Michael Pratt <mpratt@google.com>
Auto-Submit: Michael Knyszek <mknyszek@google.com>