Handle R16 conversion for POWER in the environment variables.

This patch handles the environment variables for the REAL(KIND=16)
variables like for the little/big-endian routines, so users without
who have no access to the source or are unwilling to recompile
can use this.

Syntax is, for example

GFORTRAN_CONVERT_UNIT="r16_ieee:10;little_endian:10" ./a.out

libgfortran/ChangeLog:

	* runtime/environ.c (R16_IEEE): New macro.
	(R16_IBM): New macro.
	(next_token): Handle IBM R16 conversion cases.
	(push_token): Likewise.
	(mark_single): Likewise.
	(do_parse): Likewise, initialize endian.