go.sys/unix: arm uses B not JMP

Fix build.

LGTM=rsc
R=rsc
CC=golang-codereviews
https://golang.org/cl/123510043

unix/asm_freebsd_arm.s

@@ -12,16 +12,16 @@
 // The runtime may know about them.
 
 TEXT	·Syscall(SB),NOSPLIT,$0-28
-	JMP	syscall·Syscall(SB)
+	B	syscall·Syscall(SB)
 
 TEXT	·Syscall6(SB),NOSPLIT,$0-40
-	JMP	syscall·Syscall6(SB)
+	B	syscall·Syscall6(SB)
 
 TEXT	·Syscall9(SB),NOSPLIT,$0-52
-	JMP	syscall·Syscall9(SB)
+	B	syscall·Syscall9(SB)
 
 TEXT	·RawSyscall(SB),NOSPLIT,$0-28
-	JMP	syscall·RawSyscall(SB)
+	B	syscall·RawSyscall(SB)
 
 TEXT	·RawSyscall6(SB),NOSPLIT,$0-40
-	JMP	syscall·RawSyscall6(SB)
+	B	syscall·RawSyscall6(SB)

unix/asm_linux_arm.s

@@ -12,16 +12,16 @@
 // The runtime may know about them.
 
 TEXT	·Syscall(SB),NOSPLIT,$0-28
-	JMP	syscall·Syscall(SB)
+	B	syscall·Syscall(SB)
 
 TEXT	·Syscall6(SB),NOSPLIT,$0-40
-	JMP	syscall·Syscall6(SB)
+	B	syscall·Syscall6(SB)
 
 TEXT ·RawSyscall(SB),NOSPLIT,$0-28
-	JMP	syscall·RawSyscall(SB)
+	B	syscall·RawSyscall(SB)
 
 TEXT	·RawSyscall6(SB),NOSPLIT,$0-40
-	JMP	syscall·RawSyscall6(SB)
+	B	syscall·RawSyscall6(SB)
 
 TEXT ·seek(SB),NOSPLIT,$0-32
-	JMP	syscall·seek(SB)
+	B	syscall·seek(SB)

unix/asm_netbsd_arm.s

@@ -12,16 +12,16 @@
 // The runtime may know about them.
 
 TEXT	·Syscall(SB),NOSPLIT,$0-28
-	JMP	syscall·Syscall(SB)
+	B	syscall·Syscall(SB)
 
 TEXT	·Syscall6(SB),NOSPLIT,$0-40
-	JMP	syscall·Syscall6(SB)
+	B	syscall·Syscall6(SB)
 
 TEXT	·Syscall9(SB),NOSPLIT,$0-52
-	JMP	syscall·Syscall9(SB)
+	B	syscall·Syscall9(SB)
 
 TEXT	·RawSyscall(SB),NOSPLIT,$0-28
-	JMP	syscall·RawSyscall(SB)
+	B	syscall·RawSyscall(SB)
 
 TEXT	·RawSyscall6(SB),NOSPLIT,$0-40
-	JMP	syscall·RawSyscall6(SB)
+	B	syscall·RawSyscall6(SB)