Add iOS as a platform

[steeve]

Opening in draft for now.

iOS is a special variant of GOOS=darwin. In order to support the new
CC toolchain constraints that come with bazel 0.26, we need to propagate
the same OS that what bazel expects, or else CGo won't work because it
can't find a toolchain.

This needs Bazel 0.26.1 as 0.26.0 has a bug regarding this [1].

1. Fix ios, tvos and watchos arm64 constraints bazel#8522

Also, this might require gazelle to handle the ios case, too.

[steeve]

I'm wondering is it would be better that GOOS=ios be propagated down to right before Go invocation, maintaining the illusion everywhere.

[steeve]

For gazelle, which may be required as now this select doesn't work:

    deps = select({
        "@io_bazel_rules_go//go/platform:darwin": [
            "//vendor/golang.org/x/sys/unix:go_default_library",
        ],

It should be:

    deps = select({
        "@io_bazel_rules_go//go/platform:darwin": [
            "//vendor/golang.org/x/sys/unix:go_default_library",
        ],
        "@io_bazel_rules_go//go/platform:ios": [
            "//vendor/golang.org/x/sys/unix:go_default_library",
        ],

Unless we trick it somehow ?

[jayconrod]

I've started to work on a fix for this. Currently, we have a GOOS_GOARCH table of valid platforms, and everything is generated from that. I'm working on expanding this to be a wider table with struct elements. The GOOS and GOARCH may be independent from the constraint_values.

I'd rather not introduce ios as anything other than a prefix for a few platform and toolchain targets. It's not a real GOOS value. I haven't figured out how "@io_bazel_rules_go//go/platform:darwin" is going to work yet in select. Need to experiment more.

[jayconrod]

Let's discuss on #2079 instead of here.

[steeve]

Close in favor of #2090