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)