runtime: add a single-text-section fast path to findfunc

runtime: add a single-text-section fast path to findfunc

name                   old time/op  new time/op  delta
StackCopyWithStkobj-8  11.5ms ± 4%  10.7ms ± 7%  -7.10%  (p=0.000 n=10+10)

Change-Id: Ib806d732ec11f2a6cfde229fd88aff0fe68d9e7d
Reviewed-on: https://go-review.googlesource.com/c/go/+/354129
Trust: Josh Bleecher Snyder <josharian@gmail.com>
Run-TryBot: Josh Bleecher Snyder <josharian@gmail.com>
TryBot-Result: Go Bot <gobot@golang.org>
Reviewed-by: Cherry Mui <cherryyz@google.com>