cmd/dist: require _ before GOOS and GOARCH when building bootstrap
cmd/dist: require _ before GOOS and GOARCH when building bootstrap

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 <7ea35d812706d9213868749011af1ed4fa2f6aa0@google.com>
TryBot-Result: Gobot Gobot <66cb808b70d30c07676d5e946fee83fd561249e5@golang.org>
Reviewed-by: Brad Fitzpatrick <ae9783c0b0efc69cd85ab025ddd17aa44cdc4aa5@golang.org>
Reviewed-by: Russ Cox <5ad239cb8a44f659eaaee0aa1ea5b94947abe557@golang.org>

CAFxX/go,derekmarcotte/go,Cofyc/go,codestation/go,nawawi/go,cloudflare/go,codestation/go,akutz/go,polyverse-security/go,Cofyc/go,polyverse-security/go,ImJasonH/go,codestation/go,vsekhar/elastic-go,polyverse-security/go,nawawi/go,cloudflare/go,akutz/go,vsekhar/elastic-go,akutz/go,codestation/go,ImJasonH/go,CAFxX/go,josharian/go,CAFxX/go,derekmarcotte/go,nawawi/go,Cofyc/go,golang/go,josharian/go,polyverse-security/go,golang/go,golang/go,cloudflare/go,golang/go,derekmarcotte/go,derekmarcotte/go,CAFxX/go,Cofyc/go,Cofyc/go,josharian/go,golang/go,derekmarcotte/go,ImJasonH/go,vsekhar/elastic-go,derekmarcotte/go,Cofyc/go,vsekhar/elastic-go,derekmarcotte/go,akutz/go,golang/go,akutz/go,cloudflare/go,nawawi/go,codestation/go,josharian/go,codestation/go,CAFxX/go,ImJasonH/go,CAFxX/go,cloudflare/go,cloudflare/go,golang/go,josharian/go,josharian/go,josharian/go,codestation/go,CAFxX/go,ImJasonH/go,Cofyc/go,polyverse-security/go,golang/go,CAFxX/go,akutz/go,cloudflare/go,nawawi/go,josharian/go,vsekhar/elastic-go,ImJasonH/go,nawawi/go,ImJasonH/go,akutz/go,derekmarcotte/go,vsekhar/elastic-go,vsekhar/elastic-go,christopher-henderson/Go,akutz/go,ImJasonH/go,Cofyc/go,cloudflare/go,codestation/go,polyverse-security/go,polyverse-security/go,nawawi/go,vsekhar/elastic-go,nawawi/go,polyverse-security/go