Steps for adding TLS support for a new platform:

(1) Declare TLS variant in machine/types.h by defining either
__HAVE_TLS_VARIANT_I or __HAVE_TLS_VARIANT_II.

(2) crt0.o has to call _rtld_tls_static_setup() if _DYNAMIC == NULL.
This part is already done if the new src/lib/csu/arch layout is used
by the architecture.

(3) _lwp_makecontext has to set the reserved register or kernel transfer
variable in uc_mcontext to the provided value of 'private'.

This is not possible on the VAX as there is no free space in ucontext_t.
This requires either a special version of _lwp_create or versioning
everything using ucontext_t. Debug support depends on getting the data from
ucontext_t, so the second option is possibly required.

(4) _lwp_setprivate(2) has to update the same register as
_lwp_makecontext. cpu_lwp_setprivate has to call _lwp_setprivate(2) to
reflect the kernel view. cpu_switch has to update the mapping.

_lwp_setprivate is used for the initial thread, all other threads
created by libpthread use _lwp_makecontext for this purpose.

(5) Provide __tls_get_addr and possible other MD functions for dynamic
TLS offset computation. If such alternative entry points exist (currently
only i386), also add a weak reference to 0 in src/lib/libc/tls/tls.c.

The generic implementation is:

#include <sys/cdefs.h>
#include <sys/tls.h>
#include <lwp.h>

/* Weak entry is overriden by ld.elf_so for dynamic linkage */
weak_alias(__tls_get_addr, __libc__tls_get_addr)

void *
__libc__tls_get_addr(size_t idx[2])
{
	struct tls_tcb *tcb;

	tcb = _lwp_getprivate();
	return _rtld_tls_get_addr(tcb, idx[0], idx[1]);
}

XXX Document optimisations based idx[0]

(6) Implement the necessary relocation records in mdreloc.c.  There are
typically three relocation types found in dynamic binaries:

(a) R_TYPE(TLS_DTPOFF): Offset inside the module.  The common TLS code
ensures that the DTV vector points to offset 0 inside the module TLS block.
This is normally def->st_value + rela->r_addend.

(b) R_TYPE(TLS_DTPMOD): Module index.

(c) R_TYPE(TLS_TPOFF): Static TLS offset.  The code has to check whether
the static TLS offset for this module has been allocated
(defobj->tls_done) and otherwise call _rtld_tls_offset_allocate().  This
may fail if no static space is available and the object has been pulled
in via dlopen(3).

For TLS Variant I, this is typically:

def->st_value + rela->r_addend + defobj->tlsoffset + sizeof(struct tls_tcb)

e.g. the relocation doesn't include the fixed TCB.

For TLS Variant II, this is typically:

def->st_value - defobj->tlsoffset + rela->r_addend

e.g. starting offset is counting down from the TCB.

(7) Implement _lwp_getprivate_fast() in machine/mcontext.h and set
__HAVE___LWP_GETPRIVATE_FAST.

(8) Test using src/tests/lib/libc/tls.  Make sure with "objdump -R" that
t_tls_dynamic has two TPOFF relocations and h_tls_dlopen.so.1 and
libh_tls_dynamic.so.1 have both two DTPMOD and DTPOFF relocations.