asan: buffer overflow in loongarch_elf_rtype_to_howto

asan: buffer overflow in loongarch_elf_rtype_to_howto

Seen when running ld-loongarch-elf/tlsdesc-dso test.
elfxx-loongarch.c:1844:32: runtime error: index 125 out of bounds for
type 'loongarch_reloc_howto_type [124]'

So either the loongarch_howto_table needs three more
LOONGARCH_EMPTY_HOWTO entries, or loongarch_elf_rtype_to_howto should
be testing for r_type < ARRAY_SIZE (loongarch_howto_table).  I figure
it's worth wasting a little more space to get faster lookup.

	* elfxx-loongarch.c (loongarch_howto_table): Add
	LOONGARCH_EMPTY_HOWTO entries for 121..123.
	(loongarch_elf_rtype_to_howto): Don't support slow lookup.
	Assert exact table size and r_type indexing.  Omit return cast.
	(loongarch_reloc_name_lookup): Omit assertion and return cast.
	(loongarch_reloc_type_lookup): Likewise.