implement allowzero pointer attribute

closes #1953

only needed for freestanding targets.

also adds safety for `@intToPtr` when the address is zero.

test/compile_errors.zig

@@ -2,6 +2,15 @@ const tests = @import("tests.zig");
 const builtin = @import("builtin");
 
 pub fn addCases(cases: *tests.CompileErrorContext) void {
+    cases.add(
+        "@ptrToInt 0 to non optional pointer",
+        \\export fn entry() void {
+        \\    var b = @intToPtr(*i32, 0);
+        \\}
+    ,
+        "tmp.zig:2:13: error: pointer type '*i32' does not allow address zero",
+    );
+
     cases.add(
         "cast enum literal to enum but it doesn't match",
         \\const Foo = enum {