Avoid atomic for guard acquire when that is expensive

Avoid atomic for guard acquire when that is expensive

When the atomic access involves a call to __sync_synchronize
it is better to call __cxa_guard_acquire unconditionally,
since it handles the atomics too, or is a non-threaded
implementation when there is no gthread support for this target.

This fixes also a bug for the ARM EABI big-endian target,
that is, previously the wrong bit was checked.

2020-12-08  Bernd Edlinger  <bernd.edlinger@hotmail.de>

	* decl2.c: (is_atomic_expensive_p): New helper function.
	(build_atomic_load_byte): Rename to...
	(build_atomic_load_type): ... and add new parameter type.
	(get_guard_cond): Skip the atomic here if that is expensive.
	Use the correct type for the atomic load on certain targets.