elf/Makefile: Run IFUNC tests if binutils supports IFUNC

We should run IFUNC tests with --disable-multi-arch if the toolchain
supports IFUNCs.  For correctness, --disable-multi-arch must not
remove IFUNC support from the loader.

Tested on x86-64, x32 and i686 with and without --disable-multi-arch.

	* configure.ac (have-ifunc): New LIBC_CONFIG_VAR.
	* configure: Regenerated.
	* elf/Makefile: Run IFUNC tests if binutils supports IFUNC.

Reviewed-by: Tulio Magno Quites Machado Filho <tuliom@linux.ibm.com>