Fix gdbserver cross build.

Fix gdbserver cross build.

The recent libiberty patch caused issues when cross building
gdbserver.  The Makefile ends invoking the build machine's "ar"
instead of the --host version:

  ar  ./libiberty.a \
	    ./regex.o (...)

  ar: illegal option -- .
  Usage: ar [emulation options] [-]{dmpqrstx}[abcfilNoPsSuvV] [member-name] [count] archive-file file...
	 ar -M [<mri-script]

The libiberty configure script does probe for and finds an appropriate
AR.  However, gdbserver's configure does not probe for AR and
overrides the AR used in the libiberty build by explicitly passing AR
to the sub-builds.

gdb/gdbserver/
2014-06-20  Marcus Shawcroft  <marcus.shawcroft@arm.com>

	* configure.ac: Invoke. AC_CHECK_TOOL(AR, ar).
	* Makefile.in (AR, AR_FLAGS): Define.
	* configure: Regenerate.