[gensupport] PR 91255: Do not error out immediately on set_attr_alternative with...

[gensupport] PR 91255: Do not error out immediately on set_attr_alternative with define_subst

I'm trying to add a define_subst use in the arm backend but am getting many build errors complaining about:
`set_attr_alternative' is unsupported by `define_subst'

Looking at the gensupport.c code it iterates over all define_insns and errors if any of them have set_attr_alternative.

The usecase I'm targetting doesn't involve patterns with set_attr_alternative, so I would like to make the define_subst handling
more robust to only error out if the define_subst is actually attempted on a set_attr_alternative.

This patch produces the error only if the set_attr_alternative attr matches the subst name.
This allows a build of the arm backend with a define_subst usage to succeed.

	PR other/91255
	* gensupport.c (has_subst_attribute): Error out on set_attr_alternative
	only if subst_name matches curr_attr string.

From-SVN: r274564