ld: Fix issue where PROVIDE overrides defined symbol
    
    In a linker script, a sequence like this:
    
      foo = ADDR (.some_section);
      bar = foo;
      PROVIDE (foo = 0);
    
    will result in 'bar = ADDR (.some_section)' and 'foo = 0', which seems
    like incorrect behaviour, foo is clearly defined elsewhere, and so the
    PROVIDE should not trigger.
    
    The problem is that an expression like this:
    
        foo = ADDR (.some_section);
    
    can't be evaluated until a late phase of the linker, due to the need
    for the section '.some_section' to have been placed, then the PROVIDE
    was being marked as being used during an earlier phase.  At the end of
    the link, both lines:
    
        foo = ADDR (.some_section);
        PROVIDE (foo = 0);
    
    are active, and this causes the final value of 'foo' to be 0.
    
    The solution proposed in this commit is that, during earlier phases of
    the linker, when we see the expression 'foo = ADDR (.some_section);',
    instead of ignoring the expression, we create a "fake" definition of
    'foo'.  The existence of this "fake" definition prevents the PROVIDE
    from being marked used, and during the final phase the real definition
    of 'foo' will replace the "fake" definition.
    
    The new test provide-6 covers the exact case described above.  The
    provide-7 test is similar to the above, but using constant
    expressions, this was never broken, but is added here to increase
    coverage.
    
    The provide-8 case also didn't fail before this commit, but I did
    manage to break this case during development of this patch.  This case
    was only covered by a mmix test before, so I've added this here to
    increase coverage.
    
    ld/ChangeLog:
    
    	* ldexp.c (exp_fold_tree_1): Rework condition underwhich provide
    	nodes are ignored in the tree walk, and move the location at which
    	we change provide nodes into provided nodes.
    	(exp_init_os): Add etree_provided.
    	* testsuite/ld-scripts/provide-6.d: New file.
    	* testsuite/ld-scripts/provide-6.t: New file.
    	* testsuite/ld-scripts/provide-7.d: New file.
    	* testsuite/ld-scripts/provide-7.t: New file.
    	* testsuite/ld-scripts/provide-8.d: New file.
    	* testsuite/ld-scripts/provide-8.t: New file.