[lldb] [Python] Do not attempt to flush() a read-only fd

Summary:
When creating a FileSP object, do not flush() the underlying file unless
it is open for writing.  Attempting to flush() a read-only fd results
in EBADF on NetBSD.

Reviewers: lawrence_danna, labath, krytarowski

Reviewed By: lawrence_danna, labath

Subscribers: lldb-commits

Differential Revision: https://reviews.llvm.org/D69320