tccgen: fix long long -> char/short cast

tccgen: fix long long -> char/short cast

This was causing assembler bugs in a tcc compiled by itself
at i386-asm.c:352 when ExprValue.v was changed to uint64_t:

    if (op->e.v == (int8_t)op->e.v)
        op->type |= OP_IM8S;

A general test case:

    #include <stdio.h>
    int main(int argc, char **argv)
    {
        long long ll = 4000;
        int i = (char)ll;
        printf("%d\n", i);
        return 0;
    }

Output was "4000", now "-96".

Also: add "asmtest2" as asmtest with tcc compiled by itself