_libc.py: add libc.dylib (osx dynamic library) to list passed to ffi.open()

I also grep'd through micropython-lib for other instances of ```ffi.open()``` and found 3 instances:
```
./_libc/_libc.py:            _h = ffi.open(n)
./re-pcre/re.py:pcre = ffi.open("libpcre.so.3")
./sqlite3/sqlite3.py:sq3 = ffi.open("libsqlite3.so.0")
```
My mac (OSX 10.7.5) has these libraries available in /usr/lib:
```
/usr/lib/libpcre.0.dylib
/usr/lib/libpcre.dylib
/usr/lib/libpcreposix.0.dylib
/usr/lib/libpcreposix.dylib
/usr/lib/libsqlite3.0.dylib
/usr/lib/libsqlite3.dylib
```
If you would like I could patch ```re.py``` and ```sqlite3.py``` so ffi can also open these libraries. Of course these files would also need a list iterating mechanism like that used in ```_libc.py``` or ```micropython/tests/unix/ffi_float.py```
I also don't know if I should include all of the *.dylib options or just one of each.