[libquadmath, patch] Add logbq() to libquadmath

[FX]

The attached patch adds logbq() to libquadmath, with code lifted from glibc.
It is made necessary by an upcoming patch for gfortran, adding full support for the IEEE modules on __float128, and requires logbq().

Bootstrapped and regtested on x86_64-apple-darwin14.
OK to commit to trunk?

FX

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