benchtests: Add acosf function to bench-math

Add acosf function to bench-math and copy acosf-inputs to benchtests.
Motivation for this patch is to prepare for upcoming libmvec new
functions.  Float and double version of libmvec functions stays
together.

acosf-inputs file generated from acos-inputs file using following
scaling formula:

f = d * (FLT_MAX/DBL_MAX)

Where d is input(double) and f is output(float).  If scaled float value
is duplicate in new input file, nextafterf() function used to find next
float value, ensuring no duplicates.

Reviewed-by: H.J. Lu <hjl.tools@gmail.com>