MIPS: support mips*64 as CPU and gnuabi64 as ABI

MIPS: support mips*64 as CPU and gnuabi64 as ABI

For MIPS64r6 ports, Debian as an example, `mipsisa64r6el` is
used as the cpu name in triple.
Let's recognize them by `mips*64*(el)`.

For 64bit Ports, like Debian's mips64el and mips64r6el ports,
`gnuabi64` is used as the abi section.
Let's use N64 abi by default for the triple with gnuabi64.