testsuite: Make standard_temp_file use invocation-specific directories

Just like standard_output_file, standard_temp_file should use multiple
directories to make the tests parallel-safe.  However,
standard_temp_file is sometimes called in some procedures that are not
test-specific.  For example, gdb_init uses it, but is called once before
all test files are ran.  Therefore, we can't organize it in a
temp/gdb.subdir/testname layout, like standard_output_file.

Because it's just meant for temporary files that don't really need to be
inspected after the test, we can just put them in a directory based on
the runtest pid.  There is always a single exp file being executed by a
particular runtest invocation at any given time, so it should be safe.

gdb/testsuite/ChangeLog:

	* lib/gdb.exp (standard_temp_file): Return a path specific to
	the runtest invocation.

gdb/testsuite/ChangeLog

@@ -1,3 +1,8 @@
+2016-02-16  Simon Marchi  <simon.marchi@ericsson.com>
+
+	* lib/gdb.exp (standard_temp_file): Return a path specific to
+	the runtest invocation.
+
 2016-02-16  Simon Marchi  <simon.marchi@ericsson.com>
 
 	* gdb.trace/save-trace.exp: Change relative path to be in the

gdb/testsuite/lib/gdb.exp

@@ -4339,13 +4339,12 @@ proc standard_output_file {basename} {
 # Return the name of a file in our standard temporary directory.
 
 proc standard_temp_file {basename} {
-    global objdir GDB_PARALLEL
-
-    if {[info exists GDB_PARALLEL]} {
-	return [make_gdb_parallel_path temp $basename]
-    } else {
-	return $basename
-    }
+    # Since a particular runtest invocation is only executing a single test
+    # file at any given time, we can use the runtest pid to build the
+    # path of the temp directory.
+    set dir [make_gdb_parallel_path temp [pid]]
+    file mkdir $dir
+    return [file join $dir $basename]
 }
 
 # Set 'testfile', 'srcfile', and 'binfile'.