Revert "sys-kernel/scx: remove llvm-r2.eclass"

I thought this would not cause any issues but it unfortunately seems
like it affected the ability for bindgen to find libclang in some
circumstances. Revert it.

This reverts commit 771a9249d4.

Closes: https://bugs.gentoo.org/976784
Signed-off-by: Violet Purcell <vi@vimproved.dev>
Part-of: https://codeberg.org/gentoo/gentoo/pulls/1087
Signed-off-by: Sam James <sam@gentoo.org>

sys-kernel/scx/scx-1.1.1.ebuild

@@ -3,9 +3,11 @@
 
 EAPI=8
 
+LLVM_COMPAT=( {16..21} )
+
 RUST_MIN_VER="1.88.0"
 
-inherit cargo linux-info
+inherit cargo llvm-r2 linux-info
 
 DESCRIPTION="sched_ext schedulers and tools"
 HOMEPAGE="https://github.com/sched-ext/scx"
@@ -40,7 +42,9 @@ BDEPEND="
 	app-misc/jq
 	dev-libs/protobuf[protoc(+)]
 	>=dev-util/bpftool-7.5.0
-	>=llvm-core/clang-17[llvm_targets_BPF(-)]
+	$(llvm_gen_dep '
+		llvm-core/clang:${LLVM_SLOT}=[llvm_targets_BPF(-)]
+	')
 "
 PDEPEND="~sys-kernel/scx-loader-${PV}"
 
@@ -61,9 +65,15 @@ QA_PREBUILT="/usr/bin/vmlinux_docify"
 
 pkg_setup() {
 	linux-info_pkg_setup
+	llvm-r2_pkg_setup
 	rust_pkg_setup
 }
 
+src_compile() {
+	local -x BPF_CLANG=clang-${LLVM_SLOT}
+	cargo_src_compile
+}
+
 src_test() {
 	# Skip broken tests in scx_mitosis and scx_utils
 	# Upstream: https://github.com/sched-ext/scx/issues/3418