runtime (gc_blocks.go): clear full size of allocation

This fixes a bug where runtime.alloc would not clear the padding from rounding the allocation up to a multiple of the block size.
The GC still has to scan this, and this could result in permanent false positives (and therefore memory leaks).

It also handles overflow in the size calculation.

builder/sizes_test.go

@@ -42,9 +42,9 @@ func TestBinarySize(t *testing.T) {
 	// This is a small number of very diverse targets that we want to test.
 	tests := []sizeTest{
 		// microcontrollers
-		{"hifive1b", "examples/echo", 3884, 280, 0, 2268},
-		{"microbit", "examples/serial", 2844, 360, 8, 2272},
-		{"wioterminal", "examples/pininterrupt", 7349, 1491, 116, 6912},
+		{"hifive1b", "examples/echo", 3896, 280, 0, 2268},
+		{"microbit", "examples/serial", 2860, 360, 8, 2272},
+		{"wioterminal", "examples/pininterrupt", 7361, 1491, 116, 6912},
 
 		// TODO: also check wasm. Right now this is difficult, because
 		// wasm binaries are run through wasm-opt and therefore the