[builtins] Prevent duplicate definitions for overridden functions

Summary:
Some architecture-specific function overrides (for example, i386/ashrdi3.S)
duplicate generic functions (in that case, ashrdi3.c). Prevent duplicate definitions
by filtering out the generic files before compiling.

Reviewers: compnerd, beanz

Subscribers: llvm-commits, mgorny

Differential Revision: https://reviews.llvm.org/D37166

llvm-svn: 312140