[ELF][MIPS] Change format of output relocations to Elf_Rel

Initially LLD generates Elf_Rel relocations for O32 ABI and Elf_Rela
relocations for N32 / N64 ABIs. In other words, format of input and
output relocations was always the same. Now LLD generates all output
relocations using Elf_Rel format only. It conforms to ABIs requirement.

The patch suggested by Alexander Richardson.

llvm-svn: 324064