travis: move IBM backwards in time

Using Ubuntu's cross-toolchains for powerpc* and s390x meant they were
depending on glibc symbols from Ubuntu 16.04. And if that host is ever
updated to a new release, the toolchains would raise the bar too.

This switches powerpc, powerpc64, and s390x to use crosstool-ng
toolchains, configured approximately like RHEL6 with kernel 2.6.32 and
glibc 2.12. This ABI level should also be compatible with Debian 7
(wheezy) and Ubuntu 12.04 (precise).

For powerpc64le, the challenge was that only glibc-2.19 officially added
support, but RHEL7 backported those changes to glibc-2.17. The backport
patches are complex and numerous, so instead of trying to push those
into crosstool-ng, this just uses glibc binaries directly from CentOS 7
and builds the toolchain manually.

This is ported from rust-lang/rust-buildbot#149.

r? @alexcrichton

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