nvk: Convert system values for gl_PointCoord and PointCoord into inputs

nvk: Convert system values for gl_PointCoord and PointCoord into inputs

Currently, any attempt to use gl_PointCoord (GLSL) or PointCoord (SPIR-V)
results in an assert in codegen:

ERROR: unknown nir_intrinsic_op load_point_coord

This patch addresses the error by converting nir_intrinsic_load_point_coord
system value into VARYING_SLOT_PNTC. The following tests now pass:

dEQP-VK.glsl.builtin_var.simple.pointcoord
dEQP-VK.glsl.builtin_var.simple.pointcoord_uniform_frag
dEQP-VK.glsl.builtin_var.simple.pointcoord_uniform_vert

Closes: https://gitlab.freedesktop.org/nouveau/mesa/-/issues/33
Part-of: <https://gitlab.freedesktop.org/mesa/mesa/-/merge_requests/24326>