Updates arch64 llvm-19 CI to use GCC15

GCS support was added to GCC in version 15, thus the rmake test for this
patch requires GCC15

Author: CrooseGit

src/ci/docker/host-aarch64/aarch64-gnu-llvm-19/Dockerfile

@@ -4,7 +4,7 @@ ARG DEBIAN_FRONTEND=noninteractive
 
 RUN apt-get update && apt-get install -y --no-install-recommends \
   bzip2 \
-  g++ \
+  g++-15 \
   make \
   ninja-build \
   file \