new port: math/libpgmath
This adds the compiler-callable math intrinsics library libpgmath. It is the
math library of the flang compiler with support for vector operations.
Patches were in large part adapted from the OpenBSD port.
Reviewed by: zeising (mentor), mat
Approved by: zeising (mentor)
Obtained from: Brian Callahan (email@example.com) with adaptations
Differential Revision: https://reviews.freebsd.org/D17034