testsuite: Make standard_temp_file use invocation-specific directories

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.