[GCC] Attribute ifunc support in clang

This patch add support for GCC attribute((ifunc("resolver"))) for
targets that use ELF as object file format. In general ifunc is a
special kind of function alias with type @gnu_indirect_function. LLVM
patch http://reviews.llvm.org/D15525

Differential Revision: http://reviews.llvm.org/D15524

git-svn-id: https://llvm.org/svn/llvm-project/cfe/trunk@265917 91177308-0d34-0410-b5e6-96231b3b80d8