scons: Add system library linker flags on LLVM 3.5.

llvn-3.5svn r197664 split out the linker flags from ldflags to
system-libs.

Signed-off-by: Vinson Lee <vlee@freedesktop.org>
r600g: fix SUMO2 pci id

0x9649 is sumo2, not sumo.

Signed-off-by: Alex Deucher <alexander.deucher@amd.com>
CC: "9.2" "10.0" <mesa-stable@lists.freedesktop.org>