runtime: add dummy getCurrentStackPointer functions

This is useful for the next commit, to get it to compile on all systems.

src/runtime/arch_386.go

@@ -9,3 +9,5 @@ const TargetBits = 32
 func align(ptr uintptr) uintptr {
 	return (ptr + 3) &^ 3
 }
+
+func getCurrentStackPointer() uintptr

src/runtime/arch_amd64.go

@@ -11,3 +11,5 @@ const TargetBits = 64
 func align(ptr uintptr) uintptr {
 	return (ptr + 15) &^ 15
 }
+
+func getCurrentStackPointer() uintptr

src/runtime/arch_arm64.go

@@ -9,3 +9,5 @@ const TargetBits = 64
 func align(ptr uintptr) uintptr {
 	return (ptr + 7) &^ 7
 }
+
+func getCurrentStackPointer() uintptr

src/runtime/arch_avr.go

@@ -12,3 +12,5 @@ func align(ptr uintptr) uintptr {
 	// No alignment necessary on the AVR.
 	return ptr
 }
+
+func getCurrentStackPointer() uintptr

src/runtime/arch_wasm.go

@@ -28,3 +28,5 @@ const wasmPageSize = 64 * 1024
 func align(ptr uintptr) uintptr {
 	return (ptr + 3) &^ 3
 }
+
+func getCurrentStackPointer() uintptr