Implement CONVERT specifier for OPEN.

This patch, based on Jakub's work, implements the CONVERT
specifier for the power-ieee128 brach.  It allows specifying
the conversion as r16_ieee,big_endian and the other way around,
based on a table.  Setting the conversion via environment
variable and via program option does not yet work.

gcc/ChangeLog:

	* flag-types.h (enum gfc_convert): Add flags for
	conversion.

gcc/fortran/ChangeLog:

	* libgfortran.h (unit_convert): Add flags.

libgfortran/ChangeLog:

	* Makefile.in: Regenerate.
	* io/file_pos.c (unformatted_backspace): Mask off
	R16 parts for convert.
	* io/inquire.c (inquire_via_unit): Add cases for
	R16 parts.
	* io/open.c (st_open): Add cases for R16 conversion.
	* io/transfer.c (unformatted_read): Adjust for R16 conversions.
	(unformatted_write): Likewise.
	(us_read): Mask of R16 bits.
	(data_transfer_init): Likewiese.
	(write_us_marker): Likewise.