runtime: fix mmap comments

We only pass lower 32 bits of file offset to asm routine.

R=r, dave, rsc
CC=golang-dev
https://golang.org/cl/6499118

src/pkg/runtime/runtime.h

@@ -728,6 +728,9 @@ void	runtime·parfordo(ParFor *desc);
 /*
  * low level C-called
  */
+// for mmap, we only pass the lower 32 bits of file offset to the 
+// assembly routine; the higher bits (if required), should be provided
+// by the assembly routine as 0.
 uint8*	runtime·mmap(byte*, uintptr, int32, int32, int32, uint32);
 void	runtime·munmap(byte*, uintptr);
 void	runtime·madvise(byte*, uintptr, int32);

src/pkg/runtime/sys_freebsd_386.s

@@ -89,7 +89,7 @@ TEXT runtime·mmap(SB),7,$32
 	MOVSL
 	MOVSL
 	MOVSL
-	MOVL	$0, AX	// top 64 bits of file offset
+	MOVL	$0, AX	// top 32 bits of file offset
 	STOSL
 	MOVL	$477, AX
 	INT	$0x80

src/pkg/runtime/sys_netbsd_386.s

@@ -68,7 +68,7 @@ TEXT runtime·mmap(SB),7,$36
 	MOVL	$0, AX
 	STOSL				// arg 6 - pad
 	MOVSL				// arg 7 - offset
-	MOVL	$0, AX			// top 64 bits of file offset
+	MOVL	$0, AX			// top 32 bits of file offset
 	STOSL
 	MOVL	$197, AX		// sys_mmap
 	INT	$0x80

src/pkg/runtime/sys_openbsd_386.s

@@ -69,7 +69,7 @@ TEXT runtime·mmap(SB),7,$36
 	MOVL	$0, AX
 	STOSL				// arg 6 - pad
 	MOVSL				// arg 7 - offset
-	MOVL	$0, AX			// top 64 bits of file offset
+	MOVL	$0, AX			// top 32 bits of file offset
 	STOSL
 	MOVL	$197, AX		// sys_mmap
 	INT	$0x80