arm64: fix register save/restore to include vector registers

Some vector registers must be preserved across calls, but this wasn't
happening on Linux and MacOS. When I added support for windows/arm64, I
saw that it required these vector registers to be preserved and assumed
this was Windows deviating from the standard calling convention. But
actually, Windows was just implementing the standard calling convention
and the bug was on Linux and MacOS.

This commit fixes the bug on Linux and MacOS and at the same time merges
the Go and assembly files as they no longer need to be separate.

src/internal/task/task_stack_arm64.go

@@ -1,4 +1,4 @@
-//go:build scheduler.tasks && arm64 && !windows
+//go:build scheduler.tasks && arm64
 
 package task
 
@@ -21,8 +21,16 @@ type calleeSavedRegs struct {
 	x27 uintptr
 	x28 uintptr
 	x29 uintptr
+	pc  uintptr // aka x30 aka LR
 
-	pc uintptr // aka x30 aka LR
+	d8  uintptr
+	d9  uintptr
+	d10 uintptr
+	d11 uintptr
+	d12 uintptr
+	d13 uintptr
+	d14 uintptr
+	d15 uintptr
 }
 
 // archInit runs architecture-specific setup for the goroutine startup.