runtime: simplify multiple text section handling in findfunc

In findfunc, we first us the relative PC to find the function's
index in functab. When we split text sections, as the external
linker may shift the sections, and the PC may not match the
(virtual) PC we used to build the functab. So the index may be
inaccurate, and we need to do a (forward or backward) linear
search to find the actual entry.

Instead of using the PC directly, we can first compute the
(pre-external-link virtual) relative PC and use that to find the
index in functab. This way, the index will be accurate and we will
not need to do the special backward linear search.

Change-Id: I8ab11c66b7a5a3d79aae00198b98780e10db27b0
Reviewed-on: https://go-review.googlesource.com/c/go/+/354873


Trust: Cherry Mui <cherryyz@google.com>
Trust: Josh Bleecher Snyder <josharian@gmail.com>
Run-TryBot: Cherry Mui <cherryyz@google.com>
TryBot-Result: Go Bot <gobot@golang.org>
Reviewed-by: Josh Bleecher Snyder <josharian@gmail.com>