[libgfortran, patch] Remove runtime clz() and ctz() bit intrisic functions

[FX]

We implement the LEADZ and TRAILZ intrinsics in terms of the built-in clz() and ctz() functions. Since these are not available for 128-bit integer types, we used to have support functions _gfortran_clz128() and _gfortran_ctz128() in libgfortran. In 2010, I applied a patch to avoid this and emit the necessary arithmetic directly from the front-end (https://gcc.gnu.org/ml/fortran/2010-09/msg00181.html). The support functions were kept in libgfortran for backward compatibility, but we can now remove them as we break the ABI.

Attached patch is bootstrapped and regtested on x86_64-apple-darwin13.6.0.
OK to commit?

FX

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