glsl: Slightly change the semantic of _LinkedShaders

glsl: Slightly change the semantic of _LinkedShaders

Previously _LinkedShaders was a compact array of the linked shaders
for each shader stage.  Now it is arranged such that each slot,
indexed by the MESA_SHADER_* defines, refers to a specific shader
stage.  As a result, some slots will be NULL.  This makes things a
little more complex in the linker, but it simplifies things in other
places.

As a side effect _NumLinkedShaders is removed.

NOTE: This may be a candidate for the 7.9 branch.  If there are other
patches that get backported to 7.9 that use _LinkedShader, this patch
should be cherry picked also.