New port, math/amath: simple command line calculator

https://gitlab.com/rnger/amath/

Features a case sensitive command line interface, internal IEEE 754
calculations with 15 significant digits, calculations with real and complex
numbers, variables and user defined functions, logarithmic and exponential
functions, trigonometric and hyperbolic function and selected mathematical
constants and rounding functions.

PR: 209905
Submitted by: cs@innolan.dk (maintainer)
Reviewed by: mat
Approved by: swills (mentor)
Differential Revision: https://reviews.freebsd.org/D8386