runtime: mark getfp as nosplit

runtime: mark getfp as nosplit

When compiling with -l, we can't take a stack split here.

Fixes #76702

Change-Id: Ieab1225c6259c7f16bb5188aa84bff615d9db2e5
Reviewed-on: https://go-review.googlesource.com/c/go/+/728060
LUCI-TryBot-Result: Go LUCI <golang-scoped@luci-project-accounts.iam.gserviceaccount.com>
Auto-Submit: Keith Randall <khr@golang.org>
Reviewed-by: Cherry Mui <cherryyz@google.com>
Reviewed-by: Keith Randall <khr@google.com>