internal/syscall/unix: fix PosixFallocate error check on freebsd

internal/syscall/unix: fix PosixFallocate error check on freebsd

The posix_fallocate syscall returns the result in r1 rather than in
errno:

> If successful, posix_fallocate() returns zero. It returns an error on failure, without
> setting errno.

Source: https://man.freebsd.org/cgi/man.cgi?query=posix_fallocate&sektion=2&n=1

Adjust the PosixFallocate wrappers on freebsd to account for that.

Also, CL 479715 used the same syscall wrapper for 386 and arm. However,
on arm the syscall argument order is different. The wrapper was
generated using mksyscall.go from the golang.org/x/sys/unix package,
adjusting the r1 check correspondingly.

Fixes #59352

Change-Id: I9a4e8e4546237010bc5e730c4988a2a476264cf4
Reviewed-on: https://go-review.googlesource.com/c/go/+/481621
Reviewed-by: Cherry Mui <cherryyz@google.com>
Run-TryBot: Tobias Klauser <tobias.klauser@gmail.com>
Reviewed-by: Yuval Pavel Zholkover <paulzhol@gmail.com>
Reviewed-by: Bryan Mills <bcmills@google.com>
TryBot-Result: Gopher Robot <gobot@golang.org>