[lldb] [Commands] Remove 'append' from 'platform file open' mode

Remove File::eOpenOptionAppend from the mode used by 'platform file
open' command.  According to POSIX, O_APPEND causes all successive
writes to be done at the end of the file.  This effectively makes
the offset argument to 'platform file write' meaningless.

Furthermore, apparently O_APPEND is not implemented reliably everywhere.
The Linux manpage for pwrite(2) suggests that Linux does respect
O_APPEND there while according to POSIX it should not, so the actual
behavior would be dependent on how the vFile:pwrite packet is
implemented on the server.

Ideally, the mode used for opening flags would be provided via options.
However, changing the default mode seems to be a reasonable intermediate
solution.

Differential Revision: https://reviews.llvm.org/D107664