runtime: mark findObject nosplit

runtime: mark findObject nosplit

findObject takes the pointer argument as uintptr. If the pointer is to
the local stack and calling findObject happens to require the stack to
be reallocated, then spanOf is called for the old pointer.

Marking findObject as nosplit fixes the issue.

Fixes #35068

Change-Id: I029d36f9c23f91812f18f98839edf02e0ba4082e
Reviewed-on: https://go-review.googlesource.com/c/go/+/202798
Run-TryBot: Cuong Manh Le <cuong.manhle.vn@gmail.com>
TryBot-Result: Gobot Gobot <gobot@golang.org>
Reviewed-by: Austin Clements <austin@google.com>