Fix to exception raised by Callable in call_once

[3246251196]

Based on MorphOS implementation: catch the potential exception in Callable, and when doing so, reset the flag allowing other threads calling the same callable to complete Callable.

Not extensive testing, only the code/test in https://en.cppreference.com/w/cpp/thread/call_once which was previously causing thread (Amiga Task) to become stuck waiting for the release of the shared resource (call_once flag).

== PS: Just updated this initial comment because a lot of detail was there explaining how you have to patch gthr-amiga-native.cpp which we no longer need to do since we have updated to 54.16 SDK.

[sba1]

Thanks! Can you please add the test also the adtools test folder?

[3246251196]

@sba1
Hi, I have added a test, added the expected output and made changes to the makefile. But, I do not have QEMU set up, so I am making those changes blindly.

[sba1]

Thanks! The test is enough for now. I think that the changes can be squashed with the previous patch that introduces this thread model though. However, this can be done later.

[3246251196]

Yes, that would be beneficial because it would then become implicit in previous versions of GCC. at the moment, this is only for GCC11. But, perhaps it is not a bad thing. This patch has not had extensive testing, after a while - if it holds up - we can squash it then.