Copy head (r256279) to stable/10 as part of the 10.0-RELEASE cycle.

Approved by: re (implicit)
Sponsored by: The FreeBSD Foundation

Include a couple of headers to ensure consistency between the prototype and
the function definition.

Clean out the leftovers from the i386_set_gsbase() TLS conversion.

Like on libthr, there is an i386_set_gsbase() stub implementation here
to avoid libc.so.5 issues. This should likely be a weak symbol and I
expect this will be fixed soon.

Approved by: re

Provide stub functions for i386_set_ldt() and i386_get_ldt() even when
compiling as an amd64 support binary. They will return EINVAL on an amd64
kernel, but this simplifies other #ifdefs that were getting a bit nasty.

No longer use _amd64_set_gsbase(). Use i386_set_gsbase() even when
building for amd64.

Attempt i386_set_gsbase() before using the user_ldt code. Unimplemented
sysarch() calls return EINVAL, not SIGSYS.. so we can trivially adapt.

When building the custom i386 libraries for use on amd64 kernels, we
can't use the i386_set_ldt() family of routines, because they are not
implemented. Instead, use the recently exposed direct access sysarch
routines for setting what %fs and %gs point to.

Use this for the i386 TLS _set_tp() routine, but only when compiling to
run as a 32 bit support binary for amd64 kernels.

Add support for TLS in statically linked programs.