preproc.c: Support multiple matches in paste_tokens

preproc.c: Support multiple matches in paste_tokens

In mmacro params head TOK_NUM should be concat'ed with
tail TOK_NUM only, otherwise the weird construction like

    %define id1      1
    %define idid1    2
    %define TOK_NUM  1
    %define TOK_ID  id

    %macro m 2
        mov eax, 1%1id%2 ; this expands to 1idid1
                         ; where idid1 expands to 2
                         ; and then to 12
    %endmacro

    m TOK_ID, TOK_NUM

issue error.

N.B. I've checked nasm-0.98.39 and it compiles this macro
perfectly well, for the record.

Reported-by: nasm64developer@users.sf.net
Signed-off-by: Cyrill Gorcunov <gorcunov@gmail.com>