libstdc++: Fix -Wshift-count-overflow warning in std::bitset

This shift only happens if the unsigned long long type is wider than
unsigned long but the compiler warns when it sees the shift, without
caring if it's reachable.

Use the preprocessor to compare the sizes and just reuse _M_to_ulong()
if sizeof(long) == sizeof(long long).

libstdc++-v3/ChangeLog:

	* include/std/bitset (_Base_bitset::_M_do_to_ullong): Avoid
	-Wshift-count-overflow warning.