mupdf on arm

[adr]

Mupdf needs to be compiled with gcc on armv7. With llvm it will
fail with a bus error. I think the problem is in the memory management
functions. I'm not attaching patches because I'm not familiar with
the ports system. I added COMPILER= ports-gcc; estdc++ to WANTILB;
lang/gcc/8,-libs>=8,<9 to LIB_DEPENDS and make plist.

adr.