Currently, dist allows GOOS and GOARCH to appear as *any* substring in
a file name when selecting source files to go into go_bootstrap. This
was necessary prior to Go 1.4, where it needed to match names like
"windows.c", but now it's gratuitously different from go/build. This
led to a bug chase to figure out why "stubs_nonlinux.go" was not being
built on non-Linux OSes.

Change shouldbuild to require an "_" before the GOOS and GOARCH in a
file name. This is still less strict than go/build, but the behavior
is much closer.

Change-Id: I580e9344a3c40d57c0721d345e911e8b4f141f5d
Reviewed-on: https://go-review.googlesource.com/40435
Run-TryBot: Austin Clements <austin@google.com>
TryBot-Result: Gobot Gobot <gobot@golang.org>
Reviewed-by: Brad Fitzpatrick <bradfitz@golang.org>
Reviewed-by: Russ Cox <rsc@golang.org>