gentoo's mips toolchain
Gentoo's mips64el toolchain(created by 'crossdev') support n32 by default.
However, I want o32.
Good news is I don't need mips64el, instead I need mipsel.
Thanks to spbecker for pointing it out to me.
I think mips64 here is the name of processor architecture.
Apparently loongson doesn't belong to that category.