Various fixes to TLS for MIPS.

Various fixes to TLS for MIPS.

- Clear the current thread's TLS pointer on exec. Previously the TLS
  pointer (and register) remain unchanged.

- Explicitly clear the TLS pointer when new threads are created.

- Make md_tls_tcb_offset per-process instead of per-thread.

  The layout of the TLS and TCB are identical for all threads in a
  process, it is only the TLS pointer values themselves that vary by
  thread.  This also makes setting md_tls_tcb_offset in
  cpu_set_user_tls() redundant with the setting in exec_setregs(), so
  only set it in exec_setregs().

Submitted by:	Alfredo Mazzinghi (1)
Sponsored by:	DARPA
Differential Revision:	https://reviews.freebsd.org/D24957