mesa: Detect and provide macros for function attributes pure and const.

mesa: Detect and provide macros for function attributes pure and const.

These are really useful hints to the compiler in the absence of link-time
optimization, and I'm going to use them in VC4.

I've made the const attribute be ATTRIBUTE_CONST unlike other function
attributes, because we have other things in the tree #defining CONST for
their own unrelated purposes.

v2: Alphabetize.

Reviewed-by: Kenneth Graunke <kenneth@whitecape.org> (v1)