[libquadmath, patch] Add logbq() to libquadmath

[FX]

> With the updated patch the test gfortran.dg/ieee/large_1.f90 compiles, but fails at run time due to the lines
> 
>  if (.not. ieee_support_underflow_control(x1)) call abort
> 
> and
> 
>  if (.not. ieee_support_underflow_control(x2)) call abort
> 
> IIRC Uros said that underflow id not supported for __float128.

Indeed. I fixed the code, but forgot to commit the testcase. Now done as revision 226665.

FX