runtime: prevent pointless jmp in amd64 and 386 memmove

6a and 8a rearrange memmove such that the fallthrough from move_1or2 to move_0 ends up being a JMP to a RET. Insert an explicit RET to prevent such silliness.

Do the same for memclr as prophylaxis.

benchmark                old ns/op     new ns/op     delta
BenchmarkMemmove1        4.59          4.13          -10.02%
BenchmarkMemmove2        4.58          4.13          -9.83%

LGTM=khr
R=golang-codereviews, dvyukov, minux, ruiu, bradfitz, khr
CC=golang-codereviews
https://golang.org/cl/120930043

src/pkg/runtime/memclr_386.s

@@ -63,6 +63,7 @@ clr_loop:
 clr_1or2:
 	MOVB	AX, (DI)
 	MOVB	AX, -1(DI)(BX*1)
+	RET
 clr_0:
 	RET
 clr_3or4:

src/pkg/runtime/memclr_amd64.s

@@ -62,6 +62,7 @@ clr_loop:
 clr_1or2:
 	MOVB	AX, (DI)
 	MOVB	AX, -1(DI)(BX*1)
+	RET
 clr_0:
 	RET
 clr_3or4:

src/pkg/runtime/memclr_plan9_386.s

@@ -32,6 +32,7 @@ clr_tail:
 clr_1or2:
 	MOVB	AX, (DI)
 	MOVB	AX, -1(DI)(BX*1)
+	RET
 clr_0:
 	RET
 clr_3or4:

src/pkg/runtime/memmove_386.s

@@ -115,6 +115,7 @@ move_1or2:
 	MOVB	-1(SI)(BX*1), CX
 	MOVB	AX, (DI)
 	MOVB	CX, -1(DI)(BX*1)
+	RET
 move_0:
 	RET
 move_3or4:

src/pkg/runtime/memmove_amd64.s

@@ -122,6 +122,7 @@ move_1or2:
 	MOVB	-1(SI)(BX*1), CX
 	MOVB	AX, (DI)
 	MOVB	CX, -1(DI)(BX*1)
+	RET
 move_0:
 	RET
 move_3or4:

src/pkg/runtime/memmove_plan9_386.s

@@ -101,6 +101,7 @@ move_1or2:
 	MOVB	-1(SI)(BX*1), CX
 	MOVB	AX, (DI)
 	MOVB	CX, -1(DI)(BX*1)
+	RET
 move_0:
 	RET
 move_3or4:

src/pkg/runtime/memmove_plan9_amd64.s

@@ -104,6 +104,7 @@ move_1or2:
 	MOVB	-1(SI)(BX*1), CX
 	MOVB	AX, (DI)
 	MOVB	CX, -1(DI)(BX*1)
+	RET
 move_0:
 	RET
 move_3or4: