[gdb/testsuite] Fix gdb.dwarf2/dw2-gas-workaround.exp

[gdb/testsuite] Fix gdb.dwarf2/dw2-gas-workaround.exp

Recently added test-case gdb.dwarf2/dw2-gas-workaround.exp:
- passes when gdb is configured using $(cd ../src; pwd)/configure, but
- fails when using ../src/configure.

Fix this by making the matching more precise:
...
-    -re -wrap "$objdir.*" {
+    -re -wrap "name_for_id = $objdir/$srcfile\r\n.*" {
...
such that we only fail on the line:
...
[symtab-create] start_subfile: name = dw2-lines.c, name_for_id = \
  /data/vries/gdb/leap-15-4/build/gdb/testsuite/dw2-lines.c^M
...

Reported-By: Carl Love <cel@us.ibm.com>