libstdc++: Give std::memory_order a fixed underlying type [PR89624]

libstdc++: Give std::memory_order a fixed underlying type [PR89624]

Prior to C++20 this enum type doesn't have a fixed underlying type,
which means it can be modified by -fshort-enums, which then means the
HLE bits are outside the range of valid values for the type.

As it has a fixed type of int in C++20 and later, do the same for
earlier standards too. This is technically a change for C++17 down,
because the implicit underlying type (without -fshort-enums) was
unsigned before. I doubt it matters in practice. That incompatibility
already exists between C++17 and C++20 and nobody has noticed or
complained. Now at least the underlying type will be int for all -std
modes.

libstdc++-v3/ChangeLog:

	PR libstdc++/89624
	* include/bits/atomic_base.h (memory_order): Use int as
	underlying type.
	* testsuite/29_atomics/atomic/89624.cc: New test.