Switching x86-64 to GNU2 TLS descriptors

[Florian Weimer]

* H. J. Lu:

> Compilers will never know since the build-time glibc is independent of
> the run-time glibc.   If compilers want to be 100% sure that the run-time
> is GNU2 TLS bug-free, they can require linkers which generate the
> GLIBC_ABI_GNU2_TLS dependency.

(Such a linker requirement could be enforced by requiring that the
linker recognizes a command option specific to GLIBC_ABI_GNU2_TLS, and
that current linkers treat as an error.)

Would such an unconditional requirement be acceptable to GCC 16?  I
don't think so.  So we'd still have to design a configure option for it.
And that requires further compiler changes.  It's also not entirely
clear how this would interact with -fuse-ld.

I just don't think option (b) is trivial from a compiler perspective.

Thanks,
Florian