To speed up the loader.Loader.toLocal() method, cache the index of the
most recently accessed object file and check that object's sym range
in toLocal() before doing a full binary search over all object symbol
ranges. This speeds up relink of kubernetes/hyperkube by about 2%, and
improves compilebench (relative to the dev.link branch) by about 5%:

name                      old time/op       new time/op       delta
LinkCompiler                    1.62s ± 8%        1.50s ± 9%  -7.21%  (p=0.000 n=20+19)
LinkWithoutDebugCompiler        1.13s ± 8%        1.09s ±12%    ~     (p=0.052 n=20+20)

name                      old user-time/op  new user-time/op  delta
LinkCompiler                    1.94s ±18%        1.97s ±16%    ~     (p=0.813 n=19+20)
LinkWithoutDebugCompiler        1.15s ±16%        1.13s ±12%    ~     (p=0.547 n=20+20)

Change-Id: Id5a8a847b533858373c0462f03972d436eda6748
Reviewed-on: https://go-review.googlesource.com/c/go/+/204337Reviewed-by: Jeremy Faller <jeremy@golang.org>
Reviewed-by: Cherry Zhang <cherryyz@google.com>