testsuite: libstdc++: Use effective-target libatomic

Test assumes libatomic.a is always available, but for some embedded
targets, there is no libatomic.a and the test thus fail.

libstdc++-v3/ChangeLog:

	* testsuite/29_atomics/atomic_float/compare_exchange_padding.cc:
	Use effective-target libatomic_available.

Signed-off-by: Torbjörn SVENSSON <torbjorn.svensson@foss.st.com>