Migrating to GOOS=custom

The Go distribution changes created for GOOS=tamago, and required by the tamago package, are part of an upstream proposal and described in its design document.

Important

These changes will be part of the future major release of tamago-go and tamago, regardless of upstream adoption.

The following Go distribution changes, against the current implementation, are introduced by the proposal:

`GOOS=tamago`	`GOOS=custom`
`GOOSPKG` points to module path	`GOOSPKG` points to package path
`runtime/goos`	`internal/runtime/goospkg`
`-ldflags "-T address"`	`GOOSPKG` `TextAddr uintpr` (or ldflags)
`runtime/goos.Hwinit0`	`internal/runtime/goospkg.InitHW0`
`runtime/goos.Hwinit1`	`internal/runtime/goospkg.InitHW1`
`runtime/goos.RamStart`	removed
`runtime/goos.RamSize`	removed
`runtime/goos.RamStackOffset`	removed
`runtime.MemRegion`	removed
`runtime.TextRegion`	removed
`runtime.CallOnG0`	`internal/runtime/goospkg·SystemStack`
`runtime.Printk`	`internal/runtime/goospkg.WriteConsole`
`os/signal.Relay`	`internal/runtime/goospkg.Relay`

The following tamago library changes, against the current implementation, are implemented to follow the proposal:

`GOOS=tamago`	`GOOS=custom`
`linkhwinit0` (build tag)	`linkinithw0` (build tag)
`linkhwinit1` (build tag)	`linkinithw1` (build tag)
`runtime/goos.RamStart`	`mem.RamStart`
`runtime/goos.RamSize`	`mem.RamSize`
`runtime/goos.RamStackOffset`	`mem.RamStackOffset`
`runtime.MemRegion`	`mem.Region`
`runtime.TextRegion`	`mem.Text`
`(*CPU).GetTime`	`(*CPU).GetTime(monotonic bool)`
`runtime.CPU`	`(*CPU).Name`

All changes are implemented in the following repositories:

Platform support packages