freebsd: fix flags for opening files

Prior to this fix, the compare-outputs test suite was showing a strange
behavior, the tests always stopped between tests 6-8 and had a stack track
similar to each other.

```
Test 8/68 compare-output multiple files with private function (ReleaseSmall)...OK
/usr/home/mgxm/dev/zig/zig-cache/source.zig:7:2: error: invalid token: '&'
}&(getStdOut() catch unreachable).outStream().stream;
 ^
The following command exited with error code 1:

```

With the wrong O_* flags, the source code was being written in append mode which
resulted in an invalid file

```zig
use @import("foo.zig");
use @import("bar.zig");

pub fn main() void {
	foo_function();
	bar_function();
}&(getStdOut() catch unreachable).outStream().stream;
	stdout.print("OK 2\n") catch unreachable;
}

fn privateFunction() void {
	printText();
}
```

0 files changed, 0 insertions, 0 deletions