Don't assume File-I/O mode bits match the host's format

Don't assume File-I/O mode bits match the host's format

inf_child_fileio_open and its gdbserver equivalent both assume that
the mode_t bits defined in gdb/fileio.h are the same as those used
by the open system call, but there is no mechanism to ensure this is
the case.  This commit adds a conversion function to handle systems
where the File-I/O definitions do not align with the host's.

gdb/ChangeLog:

	* common/fileio.h (fileio_to_host_mode): New declaration.
	* common/fileio.c (fileio_to_host_mode): New Function.
	* inf-child.c (inf_child_fileio_open): Process mode argument
	with fileio_to_host_mode.

gdb/gdbserver/ChangeLog:

	* hostio.c (handle_open): Process mode argument with
	fileio_to_host_mode.