Record bitset attributes internally as a list.

	Record bitset attributes internally as a list.
	Record rtx attribute values internally as ((rtx-expr)).
	* attr.scm (bitset-attr?): Delete, unused.
	(<bitset-attribute> parse-value): Rewrite.
	(/attr-parse): Rewrite bitset default value processing.
	(/attr-read): Pick out values of scalar attributes to distinguish
	them from bitset values which are a list.
	Fix spelling errors for processing of default values.
	Handle string attributes.
	(bitset-attr->list): Delete, all callers updated.
	(/bitset-attr->charmask): Renamed from bitset-attr->charmask.
	All callers updated.
	(atlist-source-form): Rewrite.
	* hardware.scm (<hardware-base> 'get-isas): Update recognition
	of all isas.
	(hardware-builtin!): Update spec of ISA attribute, (ISA foo,bar)
	-> (ISA foo bar).
	* intrinsics.scm (target:belongs-to-group?): Update, bitset attribute
	values are now lists.
	* mach.scm (def-isa-attr!): Update, bitset attribute values,
	including the default, are now lists.
	(all-isas-attr-value): Result is now a list.
	* doc/rtl.texi: Clean up pass over attribute docs.