powerpc: Fix error message during relocation overflow

The function _itoa_word() writes characters from the higher address to
the lower address, requiring the destination string to reserve that size
before calling it.

	* sysdeps/powerpc/powerpc64/dl-machine.c (_dl_reloc_overflow):
	Reserve 16 chars to reloc_addr before calling _itoa_word.

Signed-off-by: Tulio Magno Quites Machado Filho <tuliom@linux.vnet.ibm.com>
Reviewed-by: Adhemerval Zanella <adhemerval.zanella@linaro.org>