Add atomic operation functions to improve code compatibility with Delphi XE2

[delphilite]

Hello Pierre le Riche, I have always been a big fan of FastMM4, thank you for your work over the years.
I noticed that FastMM5 cannot be compiled in XE2 environment. With reference to some other code, I have completed the missing atomic operations and modified the compatibility of the compilation instructions.
The modifications have passed simple tests under Delphi XE2 and 12. I hope this PR can be accepted.

[pleriche]

Hi,

Thank you for the kind words and the pull request. The decision regarding which versions to support in FastMM5 was not an easy one. The fact that XE2 does not have the atomic operations played a part, but I recall that there were also compiler bugs. I can't remember the details, but the fact that you got it working with XE2 suggests they probably didn't end up being an issue.

I don't currently have XE2 installed, but I'll put testing the XE2 support on my to-do list. In the meantime I'll leave the pull request open so anyone who wants XE2 support can merge it themselves.