runtime: remove unused global variable emptystring

Last runtime use was removed in https://golang.org/cl/133700043,
September 2014.

Replace plan9 syscall uses with plan9-specific variable.

Change-Id: Ifb910c021c1419a7c782959f90b054ed600d9e19
Reviewed-on: https://go-review.googlesource.com/55450Reviewed-by: Martin Möhrmann <moehrmann@google.com>
Run-TryBot: Martin Möhrmann <moehrmann@google.com>
TryBot-Result: Gobot Gobot <gobot@golang.org>

src/runtime/runtime2.go

@@ -716,7 +716,6 @@ const (
 const _TracebackMaxFrames = 100
 
 var (
-	emptystring string
 	allglen    uintptr
 	allm       *m
 	allp       [_MaxGomaxprocs + 1]*p

src/syscall/asm_plan9_386.s

@@ -38,7 +38,7 @@ TEXT	·Syscall(SB),NOSPLIT,$0-32
 	JMP	copyresult3
 	
 ok3:
-	LEAL	runtime·emptystring(SB), SI	
+	LEAL	·emptystring(SB), SI
 	
 copyresult3:
 	LEAL	err+24(FP), DI
@@ -76,7 +76,7 @@ TEXT	·Syscall6(SB),NOSPLIT,$0-44
 	JMP	copyresult4
 	
 ok4:
-	LEAL	runtime·emptystring(SB), SI
+	LEAL	·emptystring(SB), SI
 	
 copyresult4:
 	LEAL	err+36(FP), DI
@@ -143,7 +143,7 @@ TEXT ·seek(SB),NOSPLIT,$0-36
 	JMP	copyresult6
 	
 ok6:
-	LEAL	runtime·emptystring(SB), SI
+	LEAL	·emptystring(SB), SI
 	
 copyresult6:
 	LEAL	err+28(FP), DI

src/syscall/asm_plan9_amd64.s

@@ -37,7 +37,7 @@ TEXT	·Syscall(SB),NOSPLIT,$0-64
 	JMP	copyresult3
 	
 ok3:
-	LEAQ	runtime·emptystring(SB), SI	
+	LEAQ	·emptystring(SB), SI
 	
 copyresult3:
 	LEAQ	err+48(FP), DI
@@ -75,7 +75,7 @@ TEXT	·Syscall6(SB),NOSPLIT,$0-88
 	JMP	copyresult4
 	
 ok4:
-	LEAQ	runtime·emptystring(SB), SI
+	LEAQ	·emptystring(SB), SI
 	
 copyresult4:
 	LEAQ	err+72(FP), DI
@@ -141,7 +141,7 @@ TEXT ·seek(SB),NOSPLIT,$0-56
 	JMP	copyresult6
 	
 ok6:
-	LEAQ	runtime·emptystring(SB), SI
+	LEAQ	·emptystring(SB), SI
 	
 copyresult6:
 	LEAQ	err+40(FP), DI

src/syscall/asm_plan9_arm.s

@@ -10,7 +10,7 @@
 // System call support for plan9 on arm
 
 TEXT	sysresult<>(SB),NOSPLIT,$12
-	MOVW	$runtime·emptystring+0(SB), R2
+	MOVW	$·emptystring+0(SB), R2
 	CMP		$-1, R0
 	B.NE	ok
 	MOVW	R1, save-4(SP)

src/syscall/syscall_plan9.go

@@ -31,6 +31,8 @@ func (e ErrorString) Timeout() bool {
 	return e == EBUSY || e == ETIMEDOUT
 }
 
+var emptystring string
+
 // A Note is a string describing a process note.
 // It implements the os.Signal interface.
 type Note string