This refactoring prepares for version-specific builtin symbol lookups.

Bug 19331817

Change-Id: I65b46a2b35872802a249a45b2f97a1d3c1e35dfe
Reviewed-on: https://swiftshader-review.googlesource.com/2330Reviewed-by: Nicolas Capens <capn@google.com>
Tested-by: Nicolas Capens <capn@google.com>