libstdc++: Fix undefined shift when _Atomic_word is 64-bit

libstdc++: Fix undefined shift when _Atomic_word is 64-bit

The check for _Atomic_word being 32-bit is just a normal runtime
condition for C++11 and C++14, because it doesn't use if-constexpr. That
means the 1LL << (CHAR_BIT * sizeof(_Atomic_word)) expression expands to
1LL << 64 on Solaris, which is ill-formed.

This adds another indirection so that the shift width is zero if the
code is unreachable.

libstdc++-v3/ChangeLog:

	* include/bits/shared_ptr_base.h (_Sp_counted_base::_M_release()):
	Make shift width conditional on __double_word condition.