During bootstrapping, cmd/dist writes a file indicating which
GOOS/GOARCH combinations are valid, and which support cgo-enabled
builds. That information previously went into the go/build package,
but today it fits in more naturally in the internal/platform package
(which already has a number of functions indicating feature support
for GOOS/GOARCH combinations).
Moreover, as of CL 450739 the cmd/go logic for determining whether to
use cgo is somewhat more nuanced than the go/build logic: cmd/go
checks for the presence of a C compiler, whereas go/build does not
(mostly because it determines its configuration at package-init time,
and checking $PATH for a C compiler is somewhat expensive).
To simplify this situation, this change:
- consolidates the “cgo supported” check in internal/platform
(alongside many other platform-support checks) instead of making
it a one-off in go/build,
- and updates a couple of tests to use testenv.HasCGO instead of
build.Default.CgoEnabled to decide whether to test a cgo-specific
behavior.