Fix building mimalloc on armv6

The armv6 builds need a specific location for the libatomic.a file.
This commit fixes that by adding a RUSTFLAGS argument for this.

Also removed the `link-arg=-s` since this is now already done during via the release profile
And removed the CFLAGS for armv7, this is already fixed by default in the blackdex/rust-musl images.

docker/amd64/Dockerfile.alpine

@@ -44,7 +44,6 @@ ENV DEBIAN_FRONTEND=noninteractive \
 RUN mkdir -pv "${CARGO_HOME}" \
     && rustup set profile minimal
 
-ENV RUSTFLAGS='-C link-arg=-s'
 
 # Creates a dummy project used to grab dependencies
 RUN USER=root cargo new --bin /app