[Pal] re-version _r_debug from GLIBC to PAL to reduce glibc patch
_r_debug symbol is interface for debugger, like gdb, to understand
how dynamic loading is happening. So it's not used by dependent
library or executable. So there is no point to put _r_debug into GLIBC
version space for export. Put it into its own version (PAL_PRIVATE).
Then, the next patch will reduce glibc modification.
The debugger doesn't specify version, so the symbol should be referenced
as default(no version).
Signed-off-by: Isaku Yamahata <isaku.yamahata@gmail.com>