[GNUABI] Create SVML-like API

[gdonval]

Currently, sleefgnuabi follows... GNU ABI, also used by libmvec.

Problem

Problems could arise if linking is not done perfectly correctly (presence, in right position, of -lsleefgnuabi in the linker commands): any incorrect use leads to a silent fallback to libmvec. Since the ABI is identical, checking symbols is tricky.

Proposed solution

Provide a SVML-like ABI.

By using -ffast-math -ftree-vectorize -mveclibabi=svml, one can guarantee that sleef is going to be used as libmvec does not provide a SVML ABI. If linking is incorrect, it is going to fail. In addition, linking order would matter less as libmvec-provided symbols and sleefsvmlabi-provided symbols would be different.

[shibatch]

@fpetrogalli Do you have any comment on this?