LD: Always make a SEGMENT_START expression section-relative

Fix an issue with the SEGMENT_START builtin function where its result is
absolute when taken from the default supplied, and section-relative when
taken from a `-T' command-line override.  This is against documentation,
inconsistent and unexpected, and with PIE executables gives an incorrect
result with the `__executable_start' symbol.

Make the result of SEGMENT_START always section-relative then.

	ld/
	* ldexp.c (fold_binary): Always make the result of SEGMENT_START
	section-relative.
	* testsuite/ld-scripts/segment-start.d: New test.
	* testsuite/ld-scripts/segment-start.ld: New test linker script.
	* testsuite/ld-scripts/segment-start.s: New test source.
	* testsuite/ld-scripts/script.exp: Run the new test.

1 files changed, 3 insertions, 1 deletions

diff --git a/ld/ldexp.c b/ld/ldexp.c
index 4b9676e..f6446dc 100644
--- a/ld/ldexp.c
+++ b/ld/ldexp.c
@@ -534,6 +534,7 @@ fold_binary (etree_type *tree)
      operand, binary.rhs is first operand.  */
   if (expld.result.valid_p && tree->type.node_code == SEGMENT_START)
     {
+      bfd_vma value = expld.result.value;
       const char *segment_name;
       segment_type *seg;
 
@@ -551,9 +552,10 @@ fold_binary (etree_type *tree)
 		       "isn't multiple of maximum page size\n"),
 		     segment_name);
 	    seg->used = TRUE;
-	    new_rel_from_abs (seg->value);
+	    value = seg->value;
 	    break;
 	  }
+      new_rel_from_abs (value);
       return;
     }