i686-apple-darwin and other 32bit Apple targets ware demoted to tier-3 due
to lack of support from the vendor, and is no longer available from rustup
for nightly, shortly beta.

DockerOSX32 job was removed as it only tests on nightly. BuildChannelsOSX job
will continue testing on these targets on older compilers (stable and below).

libc currently denies all warnings by default. This commit denies
warnings only when libc is built in CI.