[RFC, VECTOR ABI] Allow __attribute__((vector)) in GCC by default.

[Kirill Yukhin]

Hello,
Recently vector ABI was introduced into GCC
Vector versions of math functions were incorporated in to GlibC
starting from v2.22.
Unfortunately, to get this functions work `-fopenmp'
switch must be added to compiler invocation. This is due to the fact that
vector variant of math functions generated using `omp declare simd' pragma.

There's an alternative to use __attribute__((vector)) for function. Currently
it's enabled under `-fcilkplus' switch.

To enable vectorization of loops w/ calls to math functions it is reasonable
to enable parsing of attribute vector for functions unconditionally and
change GlibC's header file not to use `omp declare simd', but use 
__attribute__((vector)) instead.

If community have no pre-denial, I'll prepare a patch for GCC
main trunk & GlibC.

--
Thanks, K