RFC: constraint attributes

[Joern Rennecke <joernr at arc dot com>]

It occured to me that one way to associate a cost with an alternative is to
attach the cost to the constraint itself; and to avoid introducing yet
another syntax, the cost modifier could be an attribute of the
define_constraint (in case of overloaded constraints, it should be on the
definition which defines the variant which drives reload in this instance).
The attribute definition would go at the end of the constraint definition
like it does for insn patterns.  So if somebody has another requirement
of constraint modification - e.g. to avoid sprinkling the md file with
'#' and '*', and to overcome their limitations WRT multi-character
constraint - this could be easily added with the same syntax.
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