libstdc++: Add missing clear_padding in __atomic_float constructor

libstdc++: Add missing clear_padding in __atomic_float constructor

For 80-bit long double we need to clear the padding bits on
construction.

libstdc++-v3/ChangeLog:

	* include/bits/atomic_base.h (__atomic_float::__atomic_float(Fp)):
	Clear padding.
	* testsuite/29_atomics/atomic_float/compare_exchange_padding.cc:
	New test.

Signed-off-by: xndcn <xndchn@gmail.com>
Reviewed-by: Jonathan Wakely <jwakely@redhat.com>