fix a comment

fix a register name

R=gri
OCL=13548
CL=13548

src/runtime/rt1_amd64_linux.c

@@ -153,7 +153,7 @@ sighandler(int32 sig, siginfo* info, void** context)
         prints("\n\n");
         
 	traceback((void *)sc->rip, (void *)sc->rsp, (void *)sc->r15);
-	tracebackothers((void*)sc->__r15);
+	tracebackothers((void*)sc->r15);
 	print_sigcontext(sc);
 
 	sys·breakpoint();

src/syscall/syscall.go

@@ -5,10 +5,7 @@
 package syscall
 
 /*
- * These calls have signatures that are independent of operating system.
- *
- * For simplicity of addressing in assembler, all integers are 64 bits
- * in these calling sequences (although it complicates some, such as pipe)
+ * Foundation of system call interface.
  */
 
 func Syscall(trap int64, a1, a2, a3 int64) (r1, r2, err int64);