[Committed][AArch64] Factor out _bfd_aarch64_resize_stubs()

[Marcus Shawcroft]

Factor out stub section resizing into a standalone function.  No 
functional change.  Committed.

/Marcus

2015-03-23  Marcus Shawcroft  <marcus.shawcroft@arm.com>

        * elfnn-aarch64.c (elfNN_aarch64_size_stubs): Factor out
        stub resize code into...
        (bfd_aarch64_resize_stubs): Define.