CMAKE_BUILD_TYPE for libcxx buildbots

Hi all,

I was looking at the libcxx buildbots that we're maintaining at Linaro
[1] and I noticed that they all default to building in Debug mode. I
think we should either
(a) Switch to Release (or RelWithDebInfo), or
(b) Specify CMAKE_BUILD_TYPE explicitly as Debug so it serves as
documentation in zorg.

What is the community's preference? Is there a reason to stick with
Debug mode for the buildbots? Note that this only affects 6 builders
(aarch64, armv8 and armv7 with/without exceptions), although I don't
mind touching other bots in zorg if need be.


[1] section "Library bots"

I believe we should use RelWithDebInfo, since that's what most people build libc++ as when they ship it. I think it makes sense to test what's closest to what we ship.