_Thread_local specifiers in Cyclotomic function

[nskeip]

This solves possible race condition (issue #5110) using C11 feature.

Text for release notes

none

[dimpase]

since when is GAP running on native Windows?

[fingolfin]

thanks for your contribution. Alas, for now we are still using C99 not C11 (I am open to reopening this decision next year, but not as a side effect for a minor PR).
That said, I also think it would be better to just move these variables into our existing TLS structure (IIRC defined in src/hpc/tls.h)

[nskeip]

thanks for your contribution. Alas, for now we are still using C99 not C11 (I am open to reopening this decision next year, but not as a side effect for a minor PR). That said, I also think it would be better to just move these variables into our existing TLS structure (IIRC defined in src/hpc/tls.h)

Thanks for the feedback! I agree that placing the variables to TLS is more appropriate.