cmd/go/internal/script: define GOOS, GOARCH, and compiler conditions using suffixes

cmd/go/internal/script: define GOOS, GOARCH, and compiler conditions using suffixes

This replaces a large set of individual GOOS and GOARCH conditions
with a smaller set of more verbose conditions. On balance, the more
uniform structure and more concise documentation seem worth the
verbosity.

For #27494.

Change-Id: I73fdf9e7180a92cb1baf5d4631aeecb26ce15181
Reviewed-on: https://go-review.googlesource.com/c/go/+/420054
TryBot-Result: Gopher Robot <gobot@golang.org>
Reviewed-by: Russ Cox <rsc@golang.org>
Run-TryBot: Bryan Mills <bcmills@google.com>
Auto-Submit: Bryan Mills <bcmills@google.com>