graphics/mesa-devel: enable LTO by default

Mesa drivers are large, so let linker optimize across files.
May improve init/load time without increasing FPS counter.

Beware of dragons on non-amd64 e.g.,

$ vkcube-xcb
Selected GPU 0: Intel(R) HD Graphics 530 (SKL GT2), type: 1
Assertion failed: (!err), function demo_prepare_buffers, file Vulkan-Tools-1.2.170/cube/cube.c, line 1344.
Abort trap

Inspired by: Arch Linux