runtime: arm64 actually has 16-byte alignment like amd64

Proof: https://godbolt.org/z/as4EM3713
Essentially, this means that there are objects on arm64 that have a
16-byte alignment and so we have to respect that when we allocate things
on the heap.

src/runtime/arch_arm64.go

@@ -9,7 +9,7 @@ const deferExtraRegs = 0
 
 // Align on word boundary.
 func align(ptr uintptr) uintptr {
-	return (ptr + 7) &^ 7
+	return (ptr + 15) &^ 15
 }
 
 func getCurrentStackPointer() uintptr {