testsuite: libstdc++ atomic_float/value_init.cc requires libatomic

testsuite: libstdc++ atomic_float/value_init.cc requires libatomic

atomic_float/value_init.cc requires libatomic for some targets, i.e., when
it tries to perform an atomic operation with a 64 bit floating point
double type on a 32 bit target.  This patch adds AIX and Darwin to the
list of targets that require the libatomic option and adds the option to
the atomic_float/value_init.cc testcase.

libstdc++-v3/ChangeLog:

2020-07-28  David Edelsohn  <dje.gcc@gmail.com>
	    Jonathan Wakely  <jwakely@redhat.com>
	    Rainer Orth  <ro@CeBiTec.Uni-Bielefeld.DE>

	* testsuite/lib/dg-options.exp (add_options_for_libatomic): Add
	target powerpc-ibm-aix* and powerpc*-*-darwin*.
	* testsuite/29_atomics/atomic_float/value_init.cc: Add options
	for libatomic.