OpenWRT builds with a toolchain that doesn't set correct entries in the ELF

OpenWRT builds with a toolchain that doesn't set correct entries in the ELF
headers, giving us no way to auto-detect the architecture. We have to
hard-code the flags that we pass to objcopy. Unfortunately, we have to do
this for all MIPS targets. This could conceivably break building the code
on other MIPS targets. If that happens, we'll deal with it at that time.