fortran: Extend -fconvert= option for ppc64le r16_ieee and r16_ibm

fortran: Extend -fconvert= option for ppc64le r16_ieee and r16_ibm

This patch on top of the previously posted option handling changes patch
allows specifying -fconvert=swap,r16_ieee etc. (but will error on it
when not on powerpc64le because in the library such swapping is only
implemented for HAVE_REAL_17).

2022-01-24  Jakub Jelinek  <jakub@redhat.com>

	* lang.opt (fconvert=): Add EnumSet property and mention also
	r16_ieee and r16_ibm arguments.
	(big-endian, little-endian, native, swap): Add Set(1) property.
	(r16_ieee, r16_ibm): New EnumValue entries with Set(2) property.
	* trans-types.cc (gfc_init_kinds): Emit gfc_fatal_error for
	-fconvert=r16_ieee or -fconvert=r16_ibm when R16_IEEE <=> R16_IBM
	conversions aren't supported.