bitfields: promote to signed int

bitfields: promote to signed int

For integer promotion with for example arithmetics or
expr_cond (x ? y : z), integral types need to be promoted
to signed if they fit.

According to latest standards, this also applies to bit-field
types taking into account their specific width.

In tcc, VT_BITFIELD set means width < original type width
Field-widths between 33 and 63 are promoted to signed long long
accordingly.

    struct { unsigned long long ullb:35; } s = { 1 };
    #define X (s.ullb - 2)

    int main (void)
    {
        long long Y = X;
        printf("%d %016llx %016llx\n", X < 0, -X, -Y);
        return 0;
    }

Results:
    GCC 4.7 : 0 0000000000000001 FFFFFFF800000001
    MSVC    : 1 0000000000000001 0000000000000001
    TCC     : 1 0000000000000001 0000000000000001

Also, gcc would promote long long bitfields of size < 32
to int as well.  Example:

    struct { unsigned long long x:20; } t = { 123 };
    /* with gcc: */ printf("%d %d\n", t.x, 456);
    /* with tcc: */ printf("%lld %d\n", t.x, 456);