proper syscall convention for arm

R=rsc
http://go/go-review/1015009

src/pkg/runtime/linux/arm/sys.s

@@ -18,8 +18,8 @@ TEXT write(SB),7,$0
     	SWI	$SYS_write
 	RET
 
-TEXT exit(SB),7,$0
-	// Exit value already in R0
+TEXT exit(SB),7,$-4
+	MOVW	0(FP), R0
 	SWI	$SYS_exit
 
 TEXT runtime·mmap(SB),7,$0

src/pkg/syscall/asm_linux_arm.s

@@ -6,9 +6,6 @@
 // System calls for arm, Linux
 //
 
-#define SYS_BASE 0x00900000
-#define SYS_syscall (SYS_BASE+113);
-
 // TODO(kaib): handle error returns
 
 // func Syscall(syscall uintptr, a1, a2, a3 uintptr) (r1, r2, err uintptr);
@@ -19,7 +16,7 @@ TEXT	syscall·Syscall(SB),7,$0
 	MOVW	8(SP), R0
 	MOVW	12(SP), R1
 	MOVW	16(SP), R2
-	SWI		$SYS_syscall
+	SWI		$0
 	MOVW	R0, 20(SP)	// r1
 	MOVW	R1, 24(SP)	// r2
 	MOVW	$0, R0
@@ -38,7 +35,7 @@ TEXT	syscall·Syscall6(SB),7,$0
 	MOVW	20(SP), R3
 	MOVW	24(SP), R4
 	MOVW	28(SP), R5
-	SWI		$SYS_syscall
+	SWI		$0
 	MOVW	R0, 32(SP)	// r1
 	MOVW	R1, 36(SP)	// r2
 	MOVW	$0, R0
@@ -52,7 +49,7 @@ TEXT syscall·RawSyscall(SB),7,$0
 	MOVW	8(SP), R0
 	MOVW	12(SP), R1
 	MOVW	16(SP), R2
-	SWI		$SYS_syscall
+	SWI		$0
 	MOVW	R0, 20(SP)	// r1
 	MOVW	R1, 24(SP)	// r2
 	MOVW	$0, R0

src/pkg/syscall/zsysnum_linux_arm.go

@@ -3,9 +3,7 @@
 package syscall
 
 const (
-	SYS_SYSCALL_BASE	= 0x900000;
-	// for thumb and eabi
-	//SYS_SYSCALL_BASE = 0;
+	SYS_SYSCALL_BASE = 0;
 
 	SYS_RESTART_SYSCALL		= (SYS_SYSCALL_BASE + 0);
 	SYS_EXIT			= (SYS_SYSCALL_BASE + 1);