i965/gen7 instanced GS support for ARB_gpu_shader5

v2:
 * Convert gl_InvocationID to a system value

No piglit regressions on HSW.

MESA_EXTENSION_OVERRIDE=GL_ARB_gpu_shader5 allows piglit
arb_gpu_shader5/execution/invocation-id-basic.shader_test
to pass.

There are known issues with invalid layout qualifiers being
allowed by the compiler. (Most are pre-existing bugs)
Piglit master has tests highlighting these issues.

Patches are available at:
git://people.freedesktop.org/~jljusten/mesa gs-inv-id-v2