[lldb] [gdb-remote] Sync vFile:open mode constants with GDB

Sync the mode constants used to drive vFile:open requests with these
used by GDB and defined for the gdb remote protocol.  This makes it
possible to use 'platform file open' after connecting to gdbremote
server (and to some degree to operate on the open file modulo other
incompatibilities).

Differential Revision: https://reviews.llvm.org/D106985