math/mpdecimal: create port

libmpdec is a complete C implementation of the General Decimal Arithmetic
Specification. The specification, written by Mike Cowlishaw from IBM,
defines a general purpose arbitrary precision data type together with
rigorously specified functions and rounding behavior. As described in the
scope section of the specification, libmpdec will - with minor restrictions -
also conform to the IEEE 754-2008 Standard for Floating-Point Arithmetic,
provided that the appropriate context parameters are set.

libmpdec++ is a complete C++ implementation of the General Decimal Arithmetic
Specification. libmpdec++ is mostly a header library around libmpdec. The
library frees users from manual memory management and has an easy API with
inline operators. libmpdec++ has a thread local context for inline operators
and other functions that use the implicit context.

WWW: https://www.bytereef.org/mpdecimal/index.html

PR: 253084
Submitted by: Stefan Krah <skrah@bytereef.org>