gdb/testsuite: avoid creating temp file in gdb/testsuite/ directory

After this commit:

  commit 33c1395cf5e9deec7733691ba32c450e5c27f757
  Date:   Fri Nov 11 15:26:46 2022 +0000

      gdb/testsuite: fix gdb.trace/unavailable-dwarf-piece.exp with Clang

The gdb.trace/unavailable-dwarf-piece.exp test script was creating a
temporary file in the build/gdb/testsuite/ directory, instead of in
the expected place in the outputs directory.

Fix this by adding a call to standard_output_file.
diff --git a/gdb/testsuite/gdb.trace/unavailable-dwarf-piece.exp b/gdb/testsuite/gdb.trace/unavailable-dwarf-piece.exp
index 13c6f38..d73b9f1 100644
--- a/gdb/testsuite/gdb.trace/unavailable-dwarf-piece.exp
+++ b/gdb/testsuite/gdb.trace/unavailable-dwarf-piece.exp
@@ -22,7 +22,7 @@
 
 standard_testfile .c -dbg.S
 
-set asm_file $srcfile2
+set asm_file [standard_output_file $srcfile2]
 set opts {}
 
 if  { [gdb_compile ${srcdir}/${subdir}/${srcfile} ${binfile}1.o \