gas: fix building tc-bpf.c on s390x

gas: fix building tc-bpf.c on s390x

char is unsigned on s390x, so there are a lot of warnings like:

    gas/config/tc-bpf.c: In function 'get_token':
    gas/config/tc-bpf.c:900:14: error: comparison is always false due to limited range of data type [-Werror=type-limits]
      900 |       if (ch == EOF || len > MAX_TOKEN_SZ)
          |              ^~

Change its type to int, like in the other similar code.

There is also:

    gas/config/tc-bpf.c:735:30: error: 'bpf_endianness' may be used uninitialized in this function [-Werror=maybe-uninitialized]
      735 |    dst, be ? size[endianness - BPF_BE16] : size[endianness - BPF_LE16]);
          |                   ~~~~~~~~~~~^~~~~~~~~~

-Wmaybe-uninitialized doesn't seem to understand the FSM; just
initialize bpf_endianness to silence it.  Add an assertion to
build_bpf_endianness() in order to catch potential bugs.