[ELF][MIPS] Use lld::elf::{read,write}* instead of llvm::support::endian::{read,write}*

This allows us to delete `using namespace llvm::support::endian` and
simplify D68323. This change adds runtime config->endianness check but
the overhead should be negligible.

Reviewed By: ruiu

Differential Revision: https://reviews.llvm.org/D68561

llvm-svn: 373884