[gdb/testsuite] Fix overlapping CUs in gdb.dwarf2/dw2-linkage-name-trust.exp

When running test-case gdb.dwarf2/dw2-linkage-name-trust.exp with target board
cc-with-gdb-index, I get:
...
(gdb) file dw2-linkage-name-trust^M
Reading symbols from dw2-linkage-name-trust...^M
warning: .gdb_index address table has a range (0x4006ac - 0x4006cc) that \
  overlaps with an earlier range, ignoring .gdb_index^M
(gdb) delete breakpoints^M
...

Fix this by compiling with nodebug.

Tested on aarch64-linux.

Approved-By: Tom Tromey <tom@tromey.com>

PR testsuite/33315
Bug: https://sourceware.org/bugzilla/show_bug.cgi?id=33315

gdb/testsuite/gdb.dwarf2/dw2-linkage-name-trust.exp

@@ -27,7 +27,7 @@ standard_testfile .S
 set executable ${testfile}
 
 if {[prepare_for_testing_full "failed to prepare" \
-	 [list $testfile c++ $testfile-main.cc {c++ debug} \
+	 [list $testfile c++ $testfile-main.cc {c++ nodebug} \
 	      $srcfile {}]]} {
     return -1
 }