runtime: load errno as signed 32-bit

the function libc_errno returns a pointer to a signed-32 bit quantity,
not a 64-bit quantity.

Fixes #27004

Change-Id: I0623835ee34fd9655532251f096022a5accb58cd
Reviewed-on: https://go-review.googlesource.com/129475
Run-TryBot: Keith Randall <khr@golang.org>
TryBot-Result: Gobot Gobot <gobot@golang.org>
Reviewed-by: Brad Fitzpatrick <bradfitz@golang.org>

src/runtime/sys_darwin_amd64.s

@@ -306,7 +306,7 @@ TEXT runtime·mmap_trampoline(SB),NOSPLIT,$0
 	CMPQ	AX, $-1
 	JNE	ok
 	CALL	libc_error(SB)
-	MOVQ	(AX), DX		// errno
+	MOVLQSX	(AX), DX		// errno
 	XORL	AX, AX
 ok:
 	MOVQ	AX, 32(BX)
@@ -371,7 +371,7 @@ TEXT runtime·kevent_trampoline(SB),NOSPLIT,$0
 	CMPQ	AX, $-1
 	JNE	ok
 	CALL	libc_error(SB)
-	MOVQ	(AX), AX		// errno
+	MOVLQSX	(AX), AX		// errno
 	NEGQ	AX			// caller wants it as a negative error code
 ok:
 	POPQ	BP

src/runtime/sys_darwin_arm64.s

@@ -70,7 +70,7 @@ TEXT runtime·mmap_trampoline(SB),NOSPLIT,$0
 	CMP	R0, R2
 	BNE	ok
 	BL libc_error(SB)
-	MOVD	(R0), R1
+	MOVW	(R0), R1
 	MOVD	$0, R0
 ok:
 	MOVD	R0, 32(R19) // ret 1 p
@@ -277,7 +277,7 @@ TEXT runtime·kevent_trampoline(SB),NOSPLIT,$0
 	CMP	R0, R2
 	BNE	ok
 	BL libc_error(SB)
-	MOVD	(R0), R0	// errno
+	MOVW	(R0), R0	// errno
 	NEG	R0, R0	// caller wants it as a negative error code
 ok:
 	RET