all: gofmt more (but vendor, testdata, and top-level test directories)

CL 294430 made packages in std and cmd modules use Go 1.17 gofmt format,
adding //go:build lines. This change applies the same formatting to some
more packages that 'go fmt' missed (e.g., syscall/js, runtime/msan), and
everything else that is easy and safe to modify in bulk.

Consider the top-level test directory, testdata, and vendor directories
out of scope, since there are many files that don't follow strict gofmt
formatting, often for intentional and legitimate reasons (testing gofmt
itself, invalid Go programs that shouldn't crash the compiler, etc.).

That makes it easy and safe to gofmt -w the .go files that are found
with gofmt -l with aforementioned directories filtered out:

	$ gofmt -l . 2>/dev/null | \
		grep -v '^test/' | \
		grep -v '/testdata/' | \
		grep -v '/vendor/' | wc -l
	      51

None of the 51 files are generated. After this change, the same command
prints 0.

For #41184.

Change-Id: Ia96ee2a0f998d6a167d4473bcad17ad09bc1d86e
Reviewed-on: https://go-review.googlesource.com/c/go/+/341009


Run-TryBot: Dmitri Shuralyov <dmitshur@golang.org>
TryBot-Result: Go Bot <gobot@golang.org>
Reviewed-by: Ian Lance Taylor <iant@golang.org>
Trust: Dmitri Shuralyov <dmitshur@golang.org>