string format pad character optimization

[jepler]

While working on #18092 I checked and this costs only about 8 bytes on samd, and might make formatting with "unusual" chars a bit faster (uint32_t fill4; declared uninitialized earlier in function):

        // Other pad characters are fairly unusual, so we'll take the hit
        // and output them 4 at a time.
        fill4 = UINT32_C(0x1010101) * (unsigned char)fill;
        pad_chars = (char *)&fill4;
        pad_size = 4;

but I didn't benchmark it or anything.