Build LLVM with support for compression

Through what seems to be an oversight we don't install libz in the
aarch64 Docker image, which causes LLVM, and in turn both rustc and
llvm-tools, to be built without support for compressing PGO and coverage
files (and probably other things too). This patch rectifies that
oversight so that aarch64 joins x86_64 in having compression support.

Since we do this for both what writes and what reads these things, it
doesn't usually manifest as an issue (beyond larger-than-necessary
files). But it does mean that a profraw file written by rustc on x86_64
cannot be read by Rust's llvm-profdata on aarch64. It shows up in other
situations too, such as if someone built rustc from source with zlib
available on aarch64, but is using llvm-tools from rustup. So it feels
worthwhile to fix.

And for Google-ability, this tends to manifest with errors like:

```
profile uses zlib compression but the profile reader was built without zlib support
```

Note that we install `zlib1g-dev`, not `lib32z1-dev` as in the x86_64
image, since we're only building for 64-bit on aarch anyway.

Author: Jon Gjengset

src/ci/docker/host-aarch64/aarch64-gnu/Dockerfile

@@ -13,6 +13,7 @@ RUN apt-get update && DEBIAN_FRONTEND=noninteractive apt-get install -y --no-ins
   sudo \
   gdb \
   libssl-dev \
+  zlib1g-dev \
   pkg-config \
   xz-utils