libstdc++: Fix aliasing violation in std::shared_ptr [PR104019]

libstdc++: Fix aliasing violation in std::shared_ptr [PR104019]

The non-atomic store that sets both reference counts to zero uses a
type-punned pointer, which has undefined behaviour. We could use memset
to write 8 bytes, but we don't actually need it to be a single store
anyway. No other thread can observe the values, that's why it's safe to
use non-atomic stores in the first place. So we can just set each count
to zero.

With -fstore-merging (which is enabled by default at -O2) GCC produces
the same code for this as for memset or the type punned store. Clang
does that store merging even at -O1.

libstdc++-v3/ChangeLog:

	PR libstdc++/104019
	* include/bits/shared_ptr_base.h (_Sp_counted_base<>::_M_release):
	Set members to zero without type punning.