make.go: fix building on FreeBSD

running gopherjs would fail on FreeBSD with errors such as:
../../go1/src/vendor/golang_org/x/net/route/address.go:31:35: undeclared
name: sysAF_LINK

This happens because in $GOROOT/src/vendor/golang_org/x/net/route the
needed constants are defined in zsys_freebsd_$ARCH.go files, where $ARCH
only exists for "386", "amd64", and "arm". So as gopherjs runs for the
"js" arch, all of the zsys_freebsd* files are ignored and the constants
are not defined.

This bug was reported in
and the "solution" eventually recommended in is to run with GOOS=linux
regardless of the host OS. Using GOOS=linux side-steps the problem since
linux does not need the problematic constants, and the GOOS value is not
supposed to matter for the generated javascript.

fixes #928

Change-Id: I20757a211ea4c671438e94e57552a0cee7a25e81
1 file changed