llvm: Fix configuration of the AArch64 build

[errordeveloper]

BPF bytecode is the target, it is independent of CPU architecture.
The configuration was originally overlooked in #3.

[errordeveloper]

There is no automated image validation yet (#5), here's is what I did to validate the LLVM image I built locally before opening this PR:

$ cat Dockerfile.test-llvm                                                                                                                  
FROM docker.io/errordeveloper/cilium-llvm:629310350b8cf33a1e426e167e1caadb6848f6c5-wip as llvm-dist

FROM ubuntu:20.04
RUN apt-get update && apt-get install -y libelf1 libmnl0 file libc-bin

COPY --from=llvm-dist /bin/llc /bin/clang /bin/
RUN /bin/llc --version
RUN /bin/clang --version

RUN echo 'int main() { return 0; }' > /tmp/test.c && clang -O2 -target bpf -c /tmp/test.c -o /tmp/test.o && file /tmp/test.o
$ cat Dockerfile.test-llvm | docker buildx build --platform linux/amd64,linux/arm64 - --progress plain --no-cache

<SNIP>

#16 [linux/amd64 stage-1 4/6] RUN /bin/llc --version
#16 0.091 LLVM (http://llvm.org/):
#16 0.091   LLVM version 10.0.0
#16 0.091   Optimized build.
#16 0.091   Default target: x86_64-unknown-linux-gnu
#16 0.091   Host CPU: skylake
#16 0.091 
#16 0.091   Registered Targets:
#16 0.091     bpf   - BPF (host endian)
#16 0.091     bpfeb - BPF (big endian)
#16 0.091     bpfel - BPF (little endian)
#16 DONE 0.1s

#17 [linux/amd64 stage-1 5/6] RUN /bin/clang --version
#17 0.095 clang version 10.0.0 (https://github.com/llvm/llvm-project.git ebfae43b898e8280eebc518e035fc5ab12778cc4)
#17 0.095 Target: x86_64-unknown-linux-gnu
#17 0.095 Thread model: posix
#17 0.095 InstalledDir: /bin
#17 DONE 0.1s

#18 [linux/amd64 stage-1 6/6] RUN echo 'int main() { return 0; }' > /tmp/tes...
#18 0.121 /tmp/test.o: ELF 64-bit LSB relocatable, eBPF, version 1 (SYSV), not stripped
#18 DONE 0.2s

<SNIP>

#11 [linux/arm64 stage-1 4/6] RUN /bin/llc --version
#11 0.223 LLVM (http://llvm.org/):
#11 0.223   LLVM version 10.0.0
#11 0.223   Optimized build.
#11 0.223   Default target: x86_64-unknown-linux-gnu
#11 0.223   Host CPU: (unknown)
#11 0.223 
#11 0.223   Registered Targets:
#11 0.223     bpf   - BPF (host endian)
#11 0.223     bpfeb - BPF (big endian)
#11 0.223     bpfel - BPF (little endian)
#11 DONE 0.3s

#12 [linux/arm64 stage-1 5/6] RUN /bin/clang --version
#12 0.258 clang version 10.0.0 (https://github.com/llvm/llvm-project.git ebfae43b898e8280eebc518e035fc5ab12778cc4)
#12 0.258 Target: x86_64-unknown-linux-gnu
#12 0.258 Thread model: posix
#12 0.258 InstalledDir: /bin
#12 DONE 0.3s

#13 [linux/arm64 stage-1 6/6] RUN echo 'int main() { return 0; }' > /tmp/tes...
#13 0.653 /tmp/test.o: ELF 64-bit LSB relocatable, eBPF, version 1 (SYSV), not stripped
#13 DONE 0.7s
$