Refactor SystemStack function for arm targets.

Removing usage of AsmFull in favor of writing inline assembly in C.

src/internal/task/task_stack_cortexm.c

@@ -0,0 +1,12 @@
+#include <stdint.h>
+
+uintptr_t SystemStack() {
+    uintptr_t sp;
+    asm volatile(
+        "mrs %0, MSP"
+        : "=r"(sp)
+        :
+        : "memory"
+    );
+    return sp;
+}

src/internal/task/task_stack_cortexm.go

@@ -8,8 +8,8 @@ package task
 // PSP, which is used for goroutines) so that goroutines do not need extra stack
 // space for interrupts.
 
+import "C"
 import (
-	"device/arm"
 	"unsafe"
 )
 
@@ -67,6 +67,6 @@ func (s *state) pause() {
 
 // SystemStack returns the system stack pointer. On Cortex-M, it is always
 // available.
-func SystemStack() uintptr {
-	return arm.AsmFull("mrs {}, MSP", nil)
-}
+//
+//export SystemStack
+func SystemStack() uintptr