ppc64: Fix clobber list item for JUMP_TO_RESTORER_BLOB

The JUMP_TO_RESTORER_BLOB assembly macro has been rewrote through the
commit '89d6b39c ppc64: pie -- Add ppc64le relocation's
processing', but the Clobber list hasn't been updated.

This doesn't generate build neither runtime errors since the Clobber
list was larger than needed but GCC 7 is now raising an error when an
assembly macro is clobbering the r2 register.

This patch fix the Clobber list to just modified registers.

Fixes: 89d6b39c ("ppc64: pie -- Add ppc64le relocation's processing")
Reported-by: Adrian Reber <adrian@lisas.de>
Signed-off-by: Laurent Dufour <ldufour@linux.vnet.ibm.com>
Signed-off-by: Pavel Emelyanov <xemul@virtuozzo.com>