finish unifying thread register handling in preparation for porting

src/thread/i386/__set_thread_area.s

@@ -0,0 +1,22 @@
+.text
+.global __set_thread_area
+.type   __set_thread_area,%function
+__set_thread_area:
+	pushl %ebx
+	movl 8(%esp),%ecx
+	movl $-1,4(%ecx)
+	movl %ecx,8(%ecx)
+	movl $0xfffff,12(%ecx)
+	movl $0x51,16(%ecx)
+	leal 4(%ecx),%ebx
+	movl $243,%eax
+	int $128
+	popl %ebx
+	testl %eax,%eax
+	jnz 1f
+	movl 4(%ecx),%ecx
+	leal 3(,%ecx,8),%ecx
+	movw %cx,%gs
+1:
+	ret
+.size __set_thread_area,.-__set_thread_area