runtime: document, clean up internal/sys

Document what the values in internal/sys mean.

Remove various special cases for arm64 in the code using StackAlign.

Delete Uintreg - it was for GOARCH=amd64p32,
which was specific to GOOS=nacl and has been retired.

This CL is part of a stack adding windows/arm64
support (#36439), intended to land in the Go 1.17 cycle.
This CL is, however, not windows/arm64-specific.
It is cleanup meant to make the port (and future ports) easier.

Change-Id: I40e8fa07b4e192298b6536b98a72a751951a4383
Reviewed-on: https://go-review.googlesource.com/c/go/+/288795
Trust: Russ Cox <rsc@golang.org>
Reviewed-by: Cherry Zhang <cherryyz@google.com>
Reviewed-by: Ian Lance Taylor <iant@golang.org>

src/runtime/runtime2.go

@@ -327,7 +327,7 @@ type gobuf struct {
 	pc   uintptr
 	g    guintptr
 	ctxt unsafe.Pointer
-	ret  sys.Uintreg
+	ret  uintptr
 	lr   uintptr
 	bp   uintptr // for framepointer-enabled architectures
 }