ubsan: nds32 undefined shift

ubsan: nds32 undefined shift

Avoid implementation defined behaviour right shift of negative values,
and undefined behaviour left shift of negative values.  While this
change might give different results in the top bit of a bfd_vma
(rightshift is 1), that doesn't matter as only the bottom 8 bits of
the relocation are used.

	* elf32-nds32.c (nds32_elf_do_9_pcrel_reloc): Calculate relocation
	using a bfd_vma type.