microblaze port

based on initial work by rdp, with heavy modifications. some features
including threads are untested because qemu app-level emulation seems
to be broken and I do not have a proper system image for testing.

src/thread/microblaze/__set_thread_area.s

@@ -0,0 +1,5 @@
+.global __set_thread_area
+.type   __set_thread_area,@function
+__set_thread_area:
+	rtsd     r15, 8
+	ori      r21, r5, 0

src/thread/microblaze/__unmapself.s

@@ -0,0 +1,8 @@
+.global __unmapself
+.type   __unmapself,@function
+__unmapself:
+	ori     r12, r0, 91
+	brki    r14, 0x8
+	ori     r12, r0, 1
+	brki    r14, 0x8
+	nop

src/thread/microblaze/clone.s

@@ -0,0 +1,33 @@
+.global __clone
+.weak clone
+.type   __clone,@function
+.type   clone,@function
+
+# r5, r6, r7, r8, r9, r10, stack
+# fn, st, fl, ar, pt, tl, ct
+# fl, st, pt, tl, ctx
+
+__clone:
+clone:
+	andi    r6, r6, -16
+	addi    r6, r6, -16
+	swi     r5, r6, 0
+	swi     r8, r6, 4
+
+	ori     r5, r7, 0
+	ori     r7, r9, 0
+	ori     r8, r10, 0
+	lwi     r9, r1, 28
+	ori     r12, r0, 120
+
+	brki    r14, 8
+	beqi	r3, 1f
+	rtsd    r15, 8
+	nop
+	
+1:	lwi     r3, r1, 0
+	lwi     r5, r1, 4
+	brald   r15, r3
+	nop
+	ori     r12, r0, 1
+	brki    r14, 8

src/thread/microblaze/syscall_cp.s

@@ -0,0 +1,19 @@
+.global __syscall_cp_asm
+.type   __syscall_cp_asm,@function
+__syscall_cp_asm:
+.global __cp_begin
+__cp_begin:
+	lwi     r5, r5, 0
+	bnei    r5, __cancel
+	addi    r12, r6, 0
+	add     r5, r7, r0
+	add     r6, r8, r0
+	add     r7, r9, r0
+	add     r8, r10, r0
+	lwi     r9, r1, 28
+	lwi     r10, r1, 32
+	brki    r14, 0x8
+.global __cp_end
+__cp_end:
+	rtsd    r15, 8
+	nop