COFF: correct function auxiliary symbol data clearing

COFF: correct function auxiliary symbol data clearing

It's unclear why the array part of the union was used there, when we're
dealing with a function. Originally, when 32-bit hosts and targets were
prevailing, the memset() in question ended up clearing the entire x_fcn,
while for 64-bit hosts/targets only x_lnnoptr would have been cleared.
Then a2c7ca15a560 ("Use stdint types in coff internal_auxent") made
things consistent, but imo in the wrong direction (and likely
unintentionally). Go back to what apparently was meant originally, using
the correct part of the union now.