cmd/dist: refactor generated cgo-support logic

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.

For #58884.
For #59500.

Change-Id: I0bb2502dba4545a3d98c9e915727382ce536a0f3
Reviewed-on: https://go-review.googlesource.com/c/go/+/483695


Auto-Submit: Bryan Mills <bcmills@google.com>
Reviewed-by: Ian Lance Taylor <iant@google.com>
TryBot-Result: Gopher Robot <gobot@golang.org>
Run-TryBot: Bryan Mills <bcmills@google.com>