elf: Add a way to check if tunable is set (BZ 27069)

The patch adds two new macros, TUNABLE_GET_DEFAULT and TUNABLE_IS_INITIALIZED,
here the former get the default value with a signature similar to
TUNABLE_GET, while the later returns whether the tunable was set by
the environment variable.

Checked on x86_64-linux-gnu.
Reviewed-by: DJ Delorie <dj@redhat.com>
Tested-by: Zhangfei Gao <zhangfei.gao@linaro.org>

elf/dl-tunable-types.h

@@ -61,6 +61,7 @@ struct _tunable
 {
   const char name[TUNABLE_NAME_MAX];	/* Internal name of the tunable.  */
   tunable_type_t type;			/* Data type of the tunable.  */
+  const tunable_val_t def;		/* The value.  */
   tunable_val_t val;			/* The value.  */
   bool initialized;			/* Flag to indicate that the tunable is
 					   initialized.  */