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

lib/libthr/arch/amd64/include/pthread_md.h

@@ -47,8 +47,8 @@ _get_curthread(void)
 {
 	struct pthread *thr;
 
-	__asm __volatile("movq %%fs:%1, %0" : "=r" (thr)
-	    : "m" (*(volatile u_long *)offsetof(struct tcb, tcb_thread)));
+	__asm __volatile("movq %%fs:%c1, %0" : "=r" (thr)
+	    : "i" (offsetof(struct tcb, tcb_thread)));
 	return (thr);
 }