[Bug d/91628] libdruntime uses glibc internal symbol on s390

[ibuclaw at gdcproject dot org]

https://gcc.gnu.org/bugzilla/show_bug.cgi?id=91628

--- Comment #18 from Iain Buclaw <ibuclaw at gdcproject dot org> ---
(In reply to Iain Buclaw from comment #17)
> I have no strong preferences, if people are wanting to go with the .S file,
> then that's fine by me, feel free to commit (or I will if you'd prefer).
> 

It would be better to prefix the name with double underscores, as it is an
internal symbol to D runtime.

CSYM(__ibmz_get_tls_offset):