Add <machine/tls.h> header to hold MD constants and helpers for TLS.

The header exports the following:

• Definition of struct tcb.
• Helpers to get/set the tcb for the current thread.
• TLS_TCB_SIZE (size of TCB)
• TLS_TCB_ALIGN (alignment of TCB)
• TLS_VARIANT_I or TLS_VARIANT_II
• TLS_DTV_OFFSET (bias of pointers in dtv[])
• TLS_TP_OFFSET (bias of "thread pointer" relative to TCB)

Note that TLS_TP_OFFSET does not account for if the unbiased thread
pointer points to the start of the TCB (arm and x86) or the end of the
TCB (MIPS, PowerPC, and RISC-V).

Note also that for amd64, the struct tcb does not include the unused
tcb_spare field included in the current structure in libthr. libthr
does not use this field, and the existing calls in libc and rtld that
allocate a TCB for amd64 assume it is the size of 3 Elf_Addr's (and
thus do not allocate room for tcb_spare).

A <sys/_tls_variant_i.h> header is used by architectures using
Variant I TLS which uses a common struct tcb.

Reviewed by: kib (older version of x86/tls.h), jrtc27
Sponsored by: The University of Cambridge, Google Inc.
Differential Revision: https://reviews.freebsd.org/D33351