Add some missing arch_..._type helpers

Add some missing arch_..._type helpers

gdbtypes provides a number of helper routines that can be called instead of
using arch_type directly to create a type of a particular kind.  This patch
adds two additional such routines that have been missing so far, to allow
creation of TYPE_CODE_DECFLOAT and TYPE_CODE_POINTER types.

The patch also changes a number of places to use the new helper routines
instead of calling arch_type directly.  No functional change intended.

gdb/ChangeLog:

	* gdbtypes.h (arch_decfloat_type): New prototype.
	(arch_pointer_type): Likewise.
	* gdbtypes.c (arch_decfloat_type): New function.
	(arch_pointer_type): Likewise.
	(gdbtypes_post_init): Use arch_decfloat_type.
	* avr-tdep.c (avr_gdbarch_init): Use arch_pointer_type.
	* ft32-tdep.c (ft32_gdbarch_init): Likewise.
	* m32c-tdep.c (make_types): Likewise.
	* rl78-tdep.c (rl78_gdbarch_init): Likewise.

Signed-off-by: Ulrich Weigand <ulrich.weigand@de.ibm.com>