Aligning tcmalloc with glibc 2.35 rseq ABI

[Florian Weimer]

* Florian Weimer:

> * Chris Kennelly:
>
>> Thanks for the heads up.
>>
>> I did have a question about whether the new protocol would introduce
>> an extra memory reference while initializing a critical section.
>>
>> * With initial-exec TLS, I can directly reference __rseq_abi.
>> * With the new ABI, I might need to ask glibc for the address of the
>> registered rseq structure in its thread data.
>
> You can write __rseq_offset to a static/hidden variable in an ELF
> constructor, and then use pretty much the same assembler sequences as
> for initial-exec TLS on most architectures.

And now I'm kind of worried that we should be using ptrdiff_t for
__rseq_offset because that's what the initial-exec relocations use. 8-/

Thanks,
Florian