Constify arch_type and friends

Constify arch_type and friends

While working on the Rust support, I happened to notice that arch_type
and related functions take "char *" arguments, where "const char *"
would be more correct.  This patch fixes this oversight.  Tested by
rebuilding.

2016-06-10  Tom Tromey  <tom@tromey.com>

	* gdbtypes.c (arch_type, arch_integer_type, arch_character_type)
	(arch_boolean_type, arch_float_type, arch_complex_type)
	(arch_flags_type, append_flags_type_field)
	(append_flags_type_flag, arch_composite_type)
	(append_composite_type_field_raw)
	(append_composite_type_field_aligned)
	(append_composite_type_field): Make "name" parameter const.
	* gdbtypes.h (arch_type, arch_integer_type, arch_character_type)
	(arch_boolean_type, arch_float_type, arch_complex_type)
	(append_composite_type_field, append_composite_type_field_aligned)
	(append_composite_type_field_raw, arch_flags_type)
	(append_flags_type_field, append_flags_type_flag): Constify.