Fix platform constants not being exposed on mips64el

[xen0n]

Just noticed this while trying to bring rustup to mips64el, which is almost certainly an oversight.

More work is needed to get rustup working on mips64el, but I'm submitting this first. More fixes to come.

[xen0n]

(I believe the Travis error is spurious, likely a HTTP 503 or 504.)

[rust-highfive]

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[alexcrichton]

Thanks for the PR! I believe though that the target arch is actually mips64 for mips64el-unknown-linux-gnu. Did you find that this fixed compilation failures for you though?

[xen0n]

Oops, I realized immediately that mips64el's arch is also mips64, this is different from mipsel... Hey, the target_arch mipsel itself is bogus, the MIPS32 targets also use mips as target_arch!

(The reason the missing libc constants seemed to not show up was a mistake in overriding deps for Cargo, I believe. I tweaked too many things at once and lost track of what caused what. Closing.)