libthr: Use <machine/tls.h> for most MD TLS details.

[jhb]

Summary

Note that on amd64 this effectively removes the unused tcb_spare field
from the end of struct tcb since the definition of struct tcb in
<x86/tls.h> does not include that field.

Sponsored by: The University of Cambridge, Google Inc.

[kib]

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

46

Is this check practically needed?

[jhb]

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

46

That's a good question. x86 does not check this and assumes that a TCB is always present.