compiler: Make is_64bit(GL_*) helper more broadly available

I'd like to use this in the prog_parameter.c code, so I need to move it
into C, make it non-static, and so on.  This probably isn't the ideal
place for it, but I couldn't think of a better one.

Acked-by: Timothy Arceri <tarceri@itsqueeze.com>

src/compiler/glsl/link_varyings.h

@@ -33,6 +33,7 @@
 
 
 #include "main/glheader.h"
+#include "program/prog_parameter.h"
 
 
 struct gl_shader_program;
@@ -161,32 +162,7 @@ private:
 
    bool is_64bit() const
    {
-      switch (this->type) {
-      case GL_DOUBLE:
-      case GL_DOUBLE_VEC2:
-      case GL_DOUBLE_VEC3:
-      case GL_DOUBLE_VEC4:
-      case GL_DOUBLE_MAT2:
-      case GL_DOUBLE_MAT2x3:
-      case GL_DOUBLE_MAT2x4:
-      case GL_DOUBLE_MAT3:
-      case GL_DOUBLE_MAT3x2:
-      case GL_DOUBLE_MAT3x4:
-      case GL_DOUBLE_MAT4:
-      case GL_DOUBLE_MAT4x2:
-      case GL_DOUBLE_MAT4x3:
-      case GL_INT64_ARB:
-      case GL_INT64_VEC2_ARB:
-      case GL_INT64_VEC3_ARB:
-      case GL_INT64_VEC4_ARB:
-      case GL_UNSIGNED_INT64_ARB:
-      case GL_UNSIGNED_INT64_VEC2_ARB:
-      case GL_UNSIGNED_INT64_VEC3_ARB:
-      case GL_UNSIGNED_INT64_VEC4_ARB:
-         return true;
-      default:
-         return false;
-      }
+      return _mesa_gl_datatype_is_64bit(this->type);
    }
 
    /**