runtime: fix error check in freebsd/386 i386_set_ldt

Update #2675

The code here was using the error check for Linux/386,
not the one for FreeBSD/386. Most of the time it worked.

Thanks to Neel Natu (FreeBSD developer) for finding this.

The s/JCC/JAE/ a few lines later is a no-op but makes the
test match the rest of the file. Why we write JAE instead of JCC
I don't know, but the two are equivalent and the file might
as well be consistent.

LGTM=bradfitz, minux
R=golang-codereviews, bradfitz, minux
CC=golang-codereviews
https://golang.org/cl/99680044

src/pkg/runtime/sys_freebsd_386.s

@@ -309,8 +309,7 @@ TEXT runtime·i386_set_ldt(SB),NOSPLIT,$16
 	MOVL	AX, 8(SP)
 	MOVL	$165, AX
 	INT	$0x80
-	CMPL	AX, $0xfffff001
-	JLS	2(PC)
+	JAE	2(PC)
 	INT	$3
 	RET
 
@@ -326,7 +325,7 @@ TEXT runtime·sysctl(SB),NOSPLIT,$28
 	MOVSL				// arg 6 - newlen
 	MOVL	$202, AX		// sys___sysctl
 	INT	$0x80
-	JCC	3(PC)
+	JAE	3(PC)
 	NEGL	AX
 	RET
 	MOVL	$0, AX