runtime: unify GC interface

Make sure every to-be-implemented GC can use the same interface. As a
result, a 1MB chunk of RAM is allocated on Unix systems on init instead
of allocating on demand.

src/runtime/arch_tinygoarm.go

@@ -12,4 +12,11 @@ const GOARCH = "arm"
 const TargetBits = 32
 
 //go:extern _heap_start
-var heapStart unsafe.Pointer
+var heapStartSymbol unsafe.Pointer
+
+var heapStart = uintptr(unsafe.Pointer(&heapStartSymbol))
+
+// Align on word boundary.
+func align(ptr uintptr) uintptr {
+	return (ptr + 3) &^ 3
+}