cmake: fix libatomic check with GCC 8

[lopsided98]

Issue #, if available:
#1199

Description of changes:

Fixes detection of libatomic with GCC 8. This solution is based on https://bugs.debian.org/cgi-bin/bugreport.cgi?bug=907277#25, which describes why the error occurs (in that case with autotools).

Some of the possible solutions are:

1. Unconditionally call enable_language(C) so that the check runs in C mode (untested)
2. Run a compile test that calls __atomic_load_8 with the right number of arguments
3. Drop the explicit libatomic test altogether and just check whether C++ atomics work when linked to libatomic

I chose option 2 because it maintained the existing semantics and seemed cleaner than using the C compiler to work around this issue.

Check all that applies:

• Did a review by yourself.
• Added proper tests to cover this PR. (If tests are not applicable, explain.)
• Checked if this PR is a breaking (APIs have been changed) change.
• Checked if this PR will not introduce cross-platform inconsistent behavior.
• Checked if this PR would require a ReadMe/Wiki update.

Check which platforms you have built SDK on to verify the correctness of this PR.

• Linux
• Windows
• Android
• MacOS
• IOS
• Other Platforms

By submitting this pull request, I confirm that you can use, modify, copy, and redistribute this contribution, under the terms of your choice.

[emersonknapp]

Hi! Will this be merged soon? This problem is affecting us on the latest Raspbian

[KaibaLopez]

Sorry for hte lack of update here but the PR was merged, closing it.