The noopt builder is configured by setting GO_GCFLAGS=-N -l, but the
test/run.go test harness doesn't look at GO_GCFLAGS when processing
"errorcheck" files, it just calls compile:

  cmdline := []string{goTool(), "tool", "compile", /* etc */}

This is working as intended, since it makes the tests more robust and
independent from the environment; errorcheck files are supposed to set
additional building flags, when needed, like in:

  // errorcheck -0 -N -l

The test/bcecheck.go test used to work on the noopt builder (even if
bce is not active on -N -l) because the test was auto-contained and
the file always compiled with optimizations enabled.

In CL 107355, a new bce test dependent on an external package
(encoding.binary) was added. On the noopt builder the external package
is built using -N -l, and this causes a test failure that broke the
noopt builder:

  https://build.golang.org/log/b2be319536285e5807ee9d66d6d0ec4d57433768

To reproduce the failure, one can do:

  $ go install -a -gcflags="-N -l" std
  $ go run run.go -- checkbce.go

This change fixes the noopt builder breakage by removing the bce test
dependency on encoding/binary by defining a local Uint64() function to
be used in the test.

Change-Id: Ife71aab662001442e715c32a0b7d758349a63ff1
Reviewed-on: https://go-review.googlesource.com/136855Reviewed-by: Brad Fitzpatrick <bradfitz@golang.org>