Add the MD parts of libthr for arm64

[andrew]

Summary

This adds the MB parts of libthr for arm64

[emaste]

lib/libthr/arch/aarch64/include/pthread_md.h

47

In D2132 lib/libc/gen/tls.c we define TLS_VARIANT_I?

[kib]

lib/libthr/arch/aarch64/include/pthread_md.h

78

Why is this needed ? get_curthread() is only called in contexts where threading library should be already initialized, AFAIR.

[andrew]

lib/libthr/arch/aarch64/include/pthread_md.h

47

It's also wrong in arm.

78

It's the same on arm, mips, and powerpc.

[andrew]

Update based on feedback

[kib]

lib/libthr/arch/aarch64/include/pthread_md.h

78

But why is it needed ? Is there something on arm64 which causes calls to get_curthread() to occur before tcb for main thread is initialized ?

Either it is not needed, probably also on other listed platforms, or we have a bug on the non-listed architectures. Or some specific arm64 quirk that needs to be commented about in the _get_curthread() function.

[andrew]

Closed by commit rS280862 (authored by @andrew).