gallium: Add ARB_gl_spirv support

gallium: Add ARB_gl_spirv support

The PIPE_CAP_GL_SPIRV capability enables ARB_gl_spirv and
ARB_spirv_extensions, and will make sure the corresponding SPIR-V
capabilities and extensions lists are initialized.

The additional PIPE_CAP_GL_SPIRV_VARIABLE_POINTERS capability enables
the support for Variable Pointers in SPIR-V shaders.  This depends on
the driver and is not mandatory for ARB_gl_spirv support.

v2: Add a PIPE_CAP for Variable Pointers.  (Marek)

Reviewed-by: Alejandro Piñeiro <apinheiro@igalia.com> [v1]