aarch64: Call all string function implementations in tests

The string function implementations implemented so far do not use any
instructions that may deviate from standard aarch64, so it is possible
for all routines to run on all armv8 hardware.  Select all
implementations in the benchmarks and tests.

	* sysdeps/aarch64/multiarch/ifunc-impl-list.c
	(__libc_ifunc_impl_list): Unconditionally select thunderx
	routine for testing.