build: Support the Fedora LLVM 11 package

Fedora is on LLVM 12, and the backward compatibility package has a
non-standard name for llvm-config.

Makefile

@@ -23,6 +23,8 @@ ifeq ($(OS), Linux)
     LLVM_CONFIG=llvm-config-11
     ifneq ($(shell which llvm-config-11 2>/dev/null),)
         LLVM_CONFIG=llvm-config-11
+    else ifneq ($(shell which llvm-config-11-64 2>/dev/null),)
+        LLVM_CONFIG=llvm-config-11-64
     else
         ifneq ($(shell llvm-config --version | grep '^11\.'),)
             LLVM_CONFIG=llvm-config