targets: Use OS=linux when GOOS=android

[zachriggle]

This avoids the issue of "android" not having any registered configurations
or syscalls / ioctls / etc, when built with GOOS=android.

This occurs when building in Google3, since --config=android_arm64 selects
the Android toolchain.

This supercedes #758 which was much more complex than it needed to be.

Per the feedback there, I've used GetTarget as the main hook point, and removed many of the changes which were only needed due to unnecessary changes in targets.go.

[zachriggle]

As to some of the concerns on #758 which were not addressed directly there:

• The _linux suffix is used for GOOS=android builds, so those files are fine
• There are many places that use runtime.GOOS directly, but most of it is fine (e.g. error messages) and all of the other cases go through GetTarget which should behave correctly
  • The specifically enumerated casese of ipc_test.go and host_linux.go do this, for example. Same for execprog.go.
  • Other cases that are host-side-only (e.g. syz-manager) will have GOOS=linux already, which is fine

[dvyukov]

Very nice!
Seems you found the right spot for the change.

The _linux suffix is used for GOOS=android builds, so those files are fine

Good, so it's not a problem