runtime: add a benchmark of FPCallers

runtime: add a benchmark of FPCallers

This allows comparing frame pointer unwinding against the default
unwinder as shown below.

goos: linux
goarch: amd64
pkg: runtime
cpu: Intel(R) Xeon(R) Platinum 8375C CPU @ 2.90GHz
                    │ callers.txt │
                    │   sec/op    │
Callers/cached-32     1.254µ ± 0%
FPCallers/cached-32   24.99n ± 0%

For #16638

Change-Id: I4dd05f82254726152ef4a5d5beceab33641e9d2b
Reviewed-on: https://go-review.googlesource.com/c/go/+/475795
TryBot-Result: Gopher Robot <gobot@golang.org>
Run-TryBot: Felix Geisendörfer <felix.geisendoerfer@datadoghq.com>
Reviewed-by: Michael Knyszek <mknyszek@google.com>
Reviewed-by: Cherry Mui <cherryyz@google.com>