runtime: fix error check in freebsd/386 i386_set_ldt
runtime: fix error check in freebsd/386 i386_set_ldt

Update issue 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://codereview.appspot.com/99680044

pombredanne/go-deleteme,pombredanne/go-deleteme,tv42/old-go,pombredanne/go-deleteme,pombredanne/go-deleteme,tv42/old-go,tv42/old-go,pombredanne/go-deleteme,pombredanne/go-deleteme,tv42/old-go,pombredanne/go-deleteme,tv42/old-go,tv42/old-go,tv42/old-go