internal/gclayout: use correct lengths

The GC bitmap length is measured in multiples of the pointer alignment.
This is equal to the pointer size on all architectures except AVR.
Replace the hardcoded lengths with lengths that are computed naturally.

Author: niaow

src/internal/gclayout/gclayout.go

@@ -13,12 +13,14 @@ const (
 	// 64-bit int => bits = 6
 	sizeBits = 4 + unsafe.Sizeof(uintptr(0))/4
 
+	ptrAlign = unsafe.Alignof(uintptr(0))
+
 	sizeShift = sizeBits + 1
 
-	NoPtrs  = Layout(uintptr(0b0<<sizeShift) | uintptr(0b1<<1) | uintptr(1))
-	Pointer = Layout(uintptr(0b1<<sizeShift) | uintptr(0b1<<1) | uintptr(1))
-	String  = Layout(uintptr(0b01<<sizeShift) | uintptr(0b10<<1) | uintptr(1))
-	Slice   = Layout(uintptr(0b001<<sizeShift) | uintptr(0b11<<1) | uintptr(1))
+	NoPtrs  = Layout((0 << sizeShift) | (1 << 1) | 1)
+	Pointer = Layout((1 << sizeShift) | ((unsafe.Sizeof(unsafe.Pointer(nil)) / ptrAlign) << 1) | 1)
+	String  = Layout((1 << sizeShift) | ((unsafe.Sizeof("") / ptrAlign) << 1) | 1)
+	Slice   = Layout((1 << sizeShift) | ((unsafe.Sizeof([]byte{}) / ptrAlign) << 1) | 1)
 )
 
 func (l Layout) AsPtr() unsafe.Pointer { return unsafe.Pointer(l) }