[patch,libgcc] Fix fixed-bit.c build warning

[Georg-Johann Lay]

Variables min_high and min_low are set but not used which cases build warnings,
fixed by this patch.

Build fine for i686-pc-linux-gnu and avr-unknown-none, the latter definitely
using this code.

Ok for trunk?

Johann


        * fixed-bit.c (SATFRACT) <FROM_TYPE = 1, TO_TYPE = 4>: Only
        declare / set min_low, min_high if TO_MODE_UNSIGNED == 0.
        (SATFRACT) <FROM_TYPE = TO_TYPE = 4>: Only declare / set min_low,
        min_high if FROM_MODE_UNSIGNED == 0 and TO_MODE_UNSIGNED == 0.