libm: Update erf, add imprecise versions of missing c++11 functions

libm: Update erf, add imprecise versions of missing c++11 functions

Provide updates to erf and erff functions.

Also add weak versions of the missing c++11 long double functions by
using taking arguments of type double.  Use of these versions will
result in a linker warning to discourage program that really need
extra precision from using them.

Note that since the c/c++ specs only guarantee that long double has
precision equal to double, code that relies on these functions having
greater precision is unportable at best and broken at worst.

Taken-from: FreeBSD