builtins: assembly routines are not static

Storage Class 3 is static storage.  These symbols need to be marked as external
(storage class 2) so that they can be referenced.  Note that this external is
not the same as ELF "external" visibility, which is indicated by DLL Storage
Class (i.e. __declspec(dllexport) or __declspec(dllimport)).

llvm-svn: 211428