This patch adds IFUNC support for arm gold backend.

This patch adds IFUNC support for arm gold backend.

This is a feature required in chromeos arm development work.

Tested:
1) Built passed all-gold on x86_64 machine
2) Tested with basic gold aarch64 ifunc unittests -
   a) global ifunc, statically/non-statically linked
   b) local ifunc, statically/non-statically linked
   c) global/local, other shared library routine mixed,
   statically/non-statically linked
   d) arm/thumb mode ifunc
   e) linking chrome browser passed