New port: devel/gllvm

gllvm provides tools for building whole-program (or whole-library) LLVM bitcode
files from an unmodified C or C++ source package. It is a drop-in replacement
for wllvm, that builds the bitcode in parallel, and is faster.

WWW: https://github.com/SRI-CSL/gllvm

Approved by: tz (mentor)
Differential Revision: https://reviews.freebsd.org/D21780