libthr _get_curthread amd64: quiet gcc -Warray-bounds

Use a constant input operand instead of a bogus memory reference to tell
the compiler about offsetof(struct tcb, tcb_thread) in the fs segment.
Otherwise gcc complains if we tell it we are reading memory offset 0x10.

Reviewed by: kib
Differential Revision: https://reviews.freebsd.org/D45828