Remove core:os dependency completely from base:runtime

base/runtime/os_specific_bsd.odin

@@ -0,0 +1,21 @@
+//+build freebsd, openbsd
+//+private
+package runtime
+
+foreign import libc "system:c"
+
+foreign libc {
+	@(link_name="write")
+	_unix_write :: proc(fd: uintptr, buf: rawptr, size: int) -> int ---
+
+	__error :: proc() -> ^i32 ---
+}
+
+_os_write :: proc "contextless" (data: []byte) -> (int, _OS_Errno) {
+	ret := _unix_write(2, raw_data(data), len(data))
+	if ret < len(data) {
+		err := __error()
+		return int(ret), _OS_Errno(err^ if err != nil else 0)
+	}
+	return int(ret), 0
+}