riscv: fix logic bug in ptr_elem_ptr

I was doing duplicate work with `elemOffset` multiplying by the abi size and then the `ptr_add` `genBinOp` also multiplying.

This led to having writes happening in the wrong place.

test/behavior/basic.zig

@@ -620,7 +620,6 @@ var global_ptr = &gdt[0];
 
 test "global constant is loaded with a runtime-known index" {
     if (builtin.zig_backend == .stage2_sparc64) return error.SkipZigTest; // TODO
-    if (builtin.zig_backend == .stage2_riscv64) return error.SkipZigTest;
 
     const S = struct {
         fn doTheTest() !void {