glsl: add SYSTEM_VALUE_VERTEX_CNT

glsl: add SYSTEM_VALUE_VERTEX_CNT

Used internally in freedreno/ir3 to calc stream-out position.  Seems
like a generic enough way to implement stream-out (using str instrs),
plus it avoids compiler warnings by sneaking in a non-enum value in
switch statements.

Signed-off-by: Rob Clark <robclark@freedesktop.org>