[arm/gcc] Don't rely on KUSER_HELPERS feature for atomics

[jbrianceau]

ARM specific CONFIG_KUSER_HELPERS feature can be disabled in Linux kernel,
and in this case, we shouldn't crash. Use gcc built-in functions instead
of arm specific code if they are available.

[grpc-kokoro]

Thanks for your pull request. The automated tests will run as soon as one of the admins verifies this change is ok for us to run on our infrastructure.

[grpc-kokoro]

Thanks for your pull request. The automated tests will run as soon as one of the admins verifies this change is ok for us to run on our infrastructure.

[xfxyjwf]

Thanks for the patch! Have you tested the change with both gcc compilers >= 4.7 and <4.7?

[jbrianceau]

We have tested the change with gcc compiler >= 4.7 and it works fine, i.e. when KUSER_HELPERS kernel feature is disabled, we do not crash anymore.
The change fallbacks with the current implementation so we didn't check with gcc compiler < 4.7

[jbrianceau]

Do you need further information about this pull request @xfxyjwf ?

[xfxyjwf]

No, this PR seems good to me. Merging now.