TLS: Use <machine/tls.h> for libc and rtld.

TLS: Use <machine/tls.h> for libc and rtld.

- Include <machine/tls.h> in MD rtld_machdep.h headers.

- Remove local definitions of TLS_* constants from rtld_machdep.h
  headers and libc using the values from <machine/tls.h> instead.

- Use _tcb_set() instead of inlined versions in MD
  allocate_initial_tls() routines in rtld.  The one exception is amd64
  whose _tcb_set() invokes the amd64_set_fsbase ifunc.  rtld cannot
  use ifuncs, so amd64 inlines the logic to optionally write to fsbase
  directly.

- Use _tcb_set() instead of _set_tp() in libc.

- Use '&_tcb_get()->tcb_dtv' instead of _get_tp() in both rtld and libc.
  This permits removing _get_tp.c from rtld.

- Use TLS_TCB_SIZE and TLS_TCB_ALIGN with allocate_tls() in MD
  allocate_initial_tls() routines in rtld.

Reviewed by:	kib, jrtc27 (earlier version)
Differential Revision:	https://reviews.freebsd.org/D33353

(cherry picked from commit 8bcdb144ebe391ce243c71caf06cf417d96ce335)