[gdb/testsuite] Add comment in exec_is_pie

Add comment to exec_is_pie explaining why readelf -d output is not used.

gdb/testsuite/ChangeLog:

2020-05-25  Tom de Vries  <tdevries@suse.de>

	* lib/gdb.exp (exec_is_pie): Add comment.

---
 gdb/testsuite/lib/gdb.exp | 3 +++
 1 file changed, 3 insertions(+)

diff --git a/gdb/testsuite/lib/gdb.exp b/gdb/testsuite/lib/gdb.exp
index 7177be941b..8e22941f0b 100644
--- a/gdb/testsuite/lib/gdb.exp
+++ b/gdb/testsuite/lib/gdb.exp
@@ -5509,6 +5509,9 @@ proc exec_is_pie { executable } {
 	return -1
     }
     set readelf_program [gdb_find_readelf]
+    # We're not testing readelf -d | grep "FLAGS_1.*Flags:.*PIE"
+    # because the PIE flag is not set by all versions of gold, see PR
+    # binutils/26039.
     set res [catch {exec $readelf_program -h $executable} output]
     if { $res != 0 } {
 	return -1