update Mesa to 17.1.0

[rezny]

Summary

Update the Mesa ports to 17.1.0 with further Makefile cleanup and some patch churn (1 patch file obsoleted, 2 new patch files, and the sed mess in patch-configure is finally gone). Use of LLVM for gallium has been restricted to platforms where it exists in base because gallium is broken when compiled with a mix of GCC and LLVM, and platforms without clang in base lack libc++ so ports clang is not a viable alternative to GCC. That change fixes runtime problems observed on PPC. LLVM is still used for bulding the libs on all platforms possible since it's only a BULID_DEP for libEGL. Build issues on ARM reported by strejda and in PR 219406 have been fixed as well as a typo in libosmesa that linimon noticed.

Test Plan

Poudriere 10.3/11.0 amd64/i386 and 11.0 armv6 OK, runtime verified on 11-stable amd64, ppc64, and armv6

[swills]

Awesome, thanks!