b/t/llvm-efi: add EFI toolchain based on LLVM

This adds a Bazel toolchain for building EFI binaries using rules_cc
with LLVM installed in the container.

It does not yet add an EFI standard library.

Change-Id: I9eb15de6f4f800ab6351607d2fb01dad3135da9f
Reviewed-on: https://review.monogon.dev/c/monogon/+/333
Reviewed-by: Sergiusz Bazanski <serge@monogon.tech>
diff --git a/build/ci/Dockerfile b/build/ci/Dockerfile
index d3f3f80..8cf7146 100644
--- a/build/ci/Dockerfile
+++ b/build/ci/Dockerfile
@@ -4,6 +4,8 @@
 	dnf -y install \
 	"@Development Tools" \
 	g++ \
+	llvm \
+	lld \
 	libuuid-devel \
 	python3 \
 	nasm \