Set mode of decimal floats before calling layout_type

Set mode of decimal floats before calling layout_type

Previously decimal floating-point types were created and laid
out as binary floating-point types, then the caller changed
the mode to a decimal mode later.  The problem with that
approach is that not all targets support an equivalent binary
floating-point mode.  When they didn't, we would give the
type BLKmode and lay it out as a zero-sized type.

This probably had no effect in practice.  If a target doesn't
support a binary mode then it's unlikely to support the decimal
equivalent either.  However, with the stricter mode checking
added by later patches, we would assert if a scalar floating-
point type didn't have a scalar floating-point mode.

gcc/
2016-11-16  Richard Sandiford  <richard.sandiford@arm.com>
	    Alan Hayward  <alan.hayward@arm.com>
	    David Sherwood  <david.sherwood@arm.com>

	* stor-layout.c (layout_type): Allow the caller to set the mode of
	a float type.  Only choose one here if the mode is still VOIDmode.
	* tree.c (build_common_tree_nodes): Set the type mode of decimal
	floats before calling layout_type.
	* config/rs6000/rs6000.c (rs6000_init_builtins): Likewise.

Co-Authored-By: Alan Hayward <alan.hayward@arm.com>
Co-Authored-By: David Sherwood <david.sherwood@arm.com>
From-SVN: r242862