In x86 arch_os_get_current_thread(), do not load from %fs

In x86 arch_os_get_current_thread(), do not load from %fs

Simplify arch_os_get_current_thread() such that %fs is not being
loaded directly even on platforms which preserve it reliably (in
particular, Linux/x86 and Solaris/x86).

Aside from a code simplification and reduction of cross-platform
differences, this change is an improvement in the following way:

If arch_os_get_current_thread() gets called in a non-Lisp pthread,
it now returns NULL cleanly even on the platforms mentioned above.

On the other platforms, keep the support for restoring %fs, but
refactor to remove code duplication with arch_os_thread_init().
Coalesce the two differently-spelled Lisp features for this case
into one.