gen: Allow unspec numbers in .md attributes

gen: Allow unspec numbers in .md attributes

Tamar pointed out that:

  (unspec:M ... <FOO>)

didn't work when a value of attribute FOO was defined by
define_constant, such as in:

  (define_int_attribute FOO [(UNSPEC_A "UNSPEC_B") ...])

This is because symbolic constants are substituted during lexing
and only apply to bare symbol names, not strings.

One option would have been to extend this lexing substitution
to define_*_attribute values as well.  However, that would replace
symbolic names with integer constants in the generated .cc code,
making it less readable.

This patch goes for the more localised approach of only
applying define_constants when we want their integer value.

I don't think any changes to the docs are needed.  This isn't
adding a new feature, it's just making an existing one work in
the expected way.

gcc/
	* read-rtl.cc (find_int): Substitute symbolic constants
	before converting the string to an integer.