This is implementing the MCP from rust-lang/compiler-team#493. It is
increasing the minimum requirements of a couple Tier 1 targets, and
others at lower tiers, so this should go through FCP sign-offs for both
`T-compiler` and `T-release`.
The new `linux-gnu` baseline is kernel 3.2 and glibc 2.17. We will also
take that kernel as the minimum floor for _all_ `*-linux-*` targets, so
it may be broadly assumed in the implementation of the standard library.
That does not preclude specific targets from having greater requirements
where it makes sense, like a new arch needing something newer, or a
platform like `linux-android` choosing a newer baseline.
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