riscv: implement sub-byte addition

test/behavior/struct.zig

@@ -1768,8 +1768,6 @@ test "struct init with no result pointer sets field result types" {
 }
 
 test "runtime side-effects in comptime-known struct init" {
-    if (builtin.zig_backend == .stage2_riscv64) return error.SkipZigTest;
-
     var side_effects: u4 = 0;
     const S = struct { a: u4, b: u4, c: u4, d: u4 };
     const init = S{