vdso, x86: Expose vdso.so.dbg through sysfs

Finding the debug version of the VDSO is not trivial as there is no common
scheme where it is placed. That's especially problematic for CI testing.

The VDSO futex unlock mechanism requires for testing to have access to the
inner labels of the unlock assembly, which are only accessible via the
debug so.

Also for general debugging purposes it's conveniant to have access to the
debug VDSO at a well defined place.

The files are placed in /sys/kernel/vdso/ and named vdso32.so.dbg,
vdso64.so.dbg, vdsox32.so.dbg.

Signed-off-by: Thomas Gleixner <tglx@linutronix.de>