Update bazel toolchains to 0.15.0
Update bazel toolchains to 0.15.0

Taken from releases.bazel.build/bazel-toolchains.html

RELNOTES: None.
PiperOrigin-RevId: 203494047

katre/bazel,akira-baruah/bazel,dslomov/bazel-windows,twitter-forks/bazel,ButterflyNetwork/bazel,ulfjack/bazel,perezd/bazel,dropbox/bazel,davidzchen/bazel,aehlig/bazel,ulfjack/bazel,akira-baruah/bazel,dropbox/bazel,aehlig/bazel,werkt/bazel,katre/bazel,aehlig/bazel,akira-baruah/bazel,werkt/bazel,bazelbuild/bazel,aehlig/bazel,katre/bazel,davidzchen/bazel,perezd/bazel,werkt/bazel,dropbox/bazel,ulfjack/bazel,cushon/bazel,aehlig/bazel,akira-baruah/bazel,ulfjack/bazel,twitter-forks/bazel,safarmer/bazel,dslomov/bazel-windows,davidzchen/bazel,cushon/bazel,perezd/bazel,safarmer/bazel,meteorcloudy/bazel,katre/bazel,twitter-forks/bazel,meteorcloudy/bazel,perezd/bazel,bazelbuild/bazel,aehlig/bazel,davidzchen/bazel,meteorcloudy/bazel,ButterflyNetwork/bazel,cushon/bazel,dslomov/bazel-windows,perezd/bazel,werkt/bazel,ButterflyNetwork/bazel,davidzchen/bazel,akira-baruah/bazel,dslomov/bazel,cushon/bazel,twitter-forks/bazel,bazelbuild/bazel,dropbox/bazel,katre/bazel,dslomov/bazel,akira-baruah/bazel,twitter-forks/bazel,ulfjack/bazel,bazelbuild/bazel,cushon/bazel,dslomov/bazel,cushon/bazel,meteorcloudy/bazel,dropbox/bazel,safarmer/bazel,dslomov/bazel,dropbox/bazel,meteorcloudy/bazel,aehlig/bazel,dslomov/bazel-windows,ulfjack/bazel,safarmer/bazel,katre/bazel,twitter-forks/bazel,ButterflyNetwork/bazel,safarmer/bazel,davidzchen/bazel,dslomov/bazel,meteorcloudy/bazel,meteorcloudy/bazel,dslomov/bazel-windows,werkt/bazel,bazelbuild/bazel,dslomov/bazel-windows,ButterflyNetwork/bazel,safarmer/bazel,ButterflyNetwork/bazel,werkt/bazel,perezd/bazel,bazelbuild/bazel,ulfjack/bazel,dslomov/bazel,dslomov/bazel,perezd/bazel,twitter-forks/bazel,davidzchen/bazel