Revert "runtime: rename getcallerfp to getfp"

Revert "runtime: rename getcallerfp to getfp"

This reverts CL 481617.

Reason for revert: breaks test build on Windows

Change-Id: Ifc1a323b0cc521e7a5a1f7de7b3da667f5fee375
Reviewed-on: https://go-review.googlesource.com/c/go/+/494377
Reviewed-by: Bryan Mills <bcmills@google.com>
Reviewed-by: Michael Knyszek <mknyszek@google.com>
Auto-Submit: Michael Pratt <mpratt@google.com>
Run-TryBot: Michael Pratt <mpratt@google.com>
TryBot-Result: Gopher Robot <gobot@golang.org>