arch/x86/intel64: Fix assumption with dummy threads

The intel64 switch implementation doesn't actually use a switch handle
per se, just the raw thread struct pointers which get stored into the
handle field.  This works fine for normally initialized threads, but
when switching out of a dummy thread at initialization, nothing has
initialized that field and the code was dumping registers into the
bottom of memory through the resulting NULL pointer.

Fix this by skipping the load of the field value and just using an
offset instead to get the struct address, which is actually slightly
faster anyway (a SUB immediate instruction vs. the load).

Actually for extra credit we could even move the switch_handle field
to the top of the thread struct and eliminate the instruction
entirely, though if we did that it's probably worth adding some
conditional code to make the switch_handle field disappear entirely.

Signed-off-by: Andy Ross <andrew.j.ross@intel.com>