[CodeGen] Add alias for cpu_dispatch function with IFunc & Fix resolver linkage type

Multi-versioned functions defined by cpu_dispatch and implemented with IFunc
can not be called outside the translation units where they are defined due to
lack of symbols. This patch add function aliases for these functions and thus
make them visible outside.

Differential Revision: https://reviews.llvm.org/D67058
Patch by Senran Zhang

llvm-svn: 371586