unix: regenerate all definitions on FreeBSD 12.1 on {386,amd64,arm64}

I just created a couple of virtual machines/QEMU instances containing
stock copies of FreeBSD 12.1-RELEASE. On these systems I ran mkall.sh,
thereby bringing the definitions in sync with that version of FreeBSD.

I also attempted to bring the GOOS=freebsd GOARCH=arm port up to date,
but was unfortunately not able to do so. There doesn't seem to be any
documentation on how to bring this port of FreeBSD up on an emulator.
The only option seems to be to use physical hardware.

unix/mkall.sh

@@ -124,7 +124,7 @@ freebsd_arm)
 freebsd_arm64)
 	mkerrors="$mkerrors -m64"
 	mksysnum="go run mksysnum.go 'https://svn.freebsd.org/base/stable/11/sys/kern/syscalls.master'"
-	mktypes="GOARCH=$GOARCH go tool cgo -godefs"
+	mktypes="GOARCH=$GOARCH go tool cgo -godefs -- -fsigned-char"
 	;;
 netbsd_386)
 	mkerrors="$mkerrors -m32"