runtime: use LLVM intrinsic to read the stack pointer

This should result in smaller code.

src/runtime/arch_arm64.go

@@ -1,7 +1,5 @@
 package runtime
 
-import "device/arm"
-
 const GOARCH = "arm64"
 
 // The bitness of the CPU (e.g. 8, 32, 64).
@@ -11,6 +9,7 @@ const TargetBits = 64
 func align(ptr uintptr) uintptr {
 	return (ptr + 7) &^ 7
 }
+
 func getCurrentStackPointer() uintptr {
-	return arm.AsmFull("mov {}, sp", nil)
+	return uintptr(stacksave())
 }