Use GCC builtins for floor functions if desired.

Use GCC builtins for floor functions if desired.

This patch is using the corresponding GCC builtin for floorf, floor,
floorl and floorf128 if the USE_FUNCTION_BUILTIN macros are defined to one
in math-use-builtins.h.

This is the case for s390 if build with at least --march=z196 --mzarch.
Otherwise the generic implementation is used.  The code of the generic
implementation is not changed.

Reviewed-by: Adhemerval Zanella <adhemerval.zanella@linaro.org>