debug/gosym: use sort.Search in findFunc

debug/gosym: use sort.Search in findFunc

Use sort.Search instead of open-coding the binary search.
This makes the code a lot easier to work on.

As a bonus, it speeds it up.

name            old time/op    new time/op    delta
115/LineToPC-8    57.4µs ± 5%    59.2µs ± 8%   +3.19%  (p=0.003 n=15+13)
115/PCToLine-8     255ns ± 1%     192ns ± 3%  -24.63%  (p=0.000 n=15+15)

Change-Id: I41da18bfb0e745c40d24e5b96e50dfdd0c3b79f5
Reviewed-on: https://go-review.googlesource.com/c/go/+/353879
Trust: Josh Bleecher Snyder <josharian@gmail.com>
Run-TryBot: Josh Bleecher Snyder <josharian@gmail.com>
Reviewed-by: Brad Fitzpatrick <bradfitz@golang.org>