[ELF2] Add support for GNU Hash section

This patch implements --hash-style command line switch.

* By default, or with "sysv" or "both" parameters, the linker generates
  a standard ELF hash section.
* With "gnu" or "both", it produces a GNU-style hash section.

That section requires the symbols in the dynamic symbol table section, which
are referenced in the GNU hash section, to be placed after not hashed ones and
to be sorted to correspond the order of hash buckets in the GNU Hash section.

The division function, as well as estimations for the section's parameters,
are just the first rough attempt and the subjects for further adjustments.

Differential Revision: http://reviews.llvm.org/D13815

llvm-svn: 251000