runtime: document, fix libc error checks on macOS

It matters whether we are calling a function that would
return a 32-bit or 64-bit -1 on error. A few sites were wrong
and this key detail was omitted from syscall/syscallX docs.

Change-Id: I48a421b6cc4d2d2b5e58f790cc947e3cb2f98940
Reviewed-on: https://go-review.googlesource.com/c/go/+/180841
Run-TryBot: Russ Cox <rsc@golang.org>
Reviewed-by: Keith Randall <khr@golang.org>

src/runtime/lookup_darwin_amd64.s

@@ -10,7 +10,7 @@ TEXT runtime·res_init_trampoline(SB),NOSPLIT,$0
 	PUSHQ	BP
 	MOVQ	SP, BP
 	CALL	libc_res_init(SB)
-	CMPQ	AX, $-1
+	CMPL	AX, $-1
 	JNE ok
 	CALL	libc_error(SB)
 ok:
@@ -28,7 +28,7 @@ TEXT runtime·res_search_trampoline(SB),NOSPLIT,$0
 	MOVQ	0(BX), DI	// arg 1 name
 	CALL	libc_res_search(SB)
 	XORL	DX, DX
-	CMPQ	AX, $-1
+	CMPL	AX, $-1
 	JNE ok
 	CALL	libc_error(SB)
 	MOVLQSX	(AX), DX	// move return from libc_error into DX

src/runtime/sys_darwin_amd64.s

@@ -369,7 +369,7 @@ TEXT runtime·kevent_trampoline(SB),NOSPLIT,$0
 	MOVQ	40(DI), R9		// arg 6 ts
 	MOVL	0(DI), DI		// arg 1 kq
 	CALL	libc_kevent(SB)
-	CMPQ	AX, $-1
+	CMPL	AX, $-1
 	JNE	ok
 	CALL	libc_error(SB)
 	MOVLQSX	(AX), AX		// errno
@@ -556,6 +556,9 @@ TEXT runtime·pthread_cond_signal_trampoline(SB),NOSPLIT,$0
 // }
 // syscall must be called on the g0 stack with the
 // C calling convention (use libcCall).
+//
+// syscall expects a 32-bit result and tests for 32-bit -1
+// to decide there was an error.
 TEXT runtime·syscall(SB),NOSPLIT,$0
 	PUSHQ	BP
 	MOVQ	SP, BP
@@ -603,6 +606,9 @@ ok:
 // }
 // syscallX must be called on the g0 stack with the
 // C calling convention (use libcCall).
+//
+// syscallX is like syscall but expects a 64-bit result
+// and tests for 64-bit -1 to decide there was an error.
 TEXT runtime·syscallX(SB),NOSPLIT,$0
 	PUSHQ	BP
 	MOVQ	SP, BP
@@ -689,6 +695,9 @@ ok:
 // }
 // syscall6 must be called on the g0 stack with the
 // C calling convention (use libcCall).
+//
+// syscall6 expects a 32-bit result and tests for 32-bit -1
+// to decide there was an error.
 TEXT runtime·syscall6(SB),NOSPLIT,$0
 	PUSHQ	BP
 	MOVQ	SP, BP
@@ -739,6 +748,9 @@ ok:
 // }
 // syscall6X must be called on the g0 stack with the
 // C calling convention (use libcCall).
+//
+// syscall6X is like syscall6 but expects a 64-bit result
+// and tests for 64-bit -1 to decide there was an error.
 TEXT runtime·syscall6X(SB),NOSPLIT,$0
 	PUSHQ	BP
 	MOVQ	SP, BP