glsl: move is_gl_identifier() to linker_util

This had started to multiply around the NIR linker. Here we move it to a
common helper shared by GLSL IR and NIR.

Reviewed-by: Alejandro Piñeiro <apinheiro@igalia.com>
Part-of: <https://gitlab.freedesktop.org/mesa/mesa/-/merge_requests/25371>

src/compiler/glsl/gl_nir_link_varyings.c

@@ -2093,15 +2093,6 @@ varying_matches_compute_packing_order(const nir_variable *var)
    }
 }
 
-/**
- * Built-in / reserved GL variables names start with "gl_"
- */
-static bool
-is_gl_identifier(const char *s)
-{
-   return s && s[0] == 'g' && s[1] == 'l' && s[2] == '_';
-}
-
 /**
  * Record the given producer/consumer variable pair in the list of variables
  * that should later be assigned locations.