CI: use lld if present (Linux only)

EXPORT_CC value can be overridden to disable lld lookup.

Makefile

@@ -65,6 +65,10 @@ DEPS = $(LLVM_EXT_OBJ) $(LIB_CRYSTAL_TARGET)
 CFLAGS += -fPIC $(if $(debug),-g -O0)
 CXXFLAGS += $(if $(debug),-g -O0)
 
+ifeq ($(shell uname -s),Linux)
+  EXPORT_CC ?= $(if $(shell command -v ld.lld),CC="cc -fuse-ld=lld")
+endif
+
 ifeq (${LLVM_CONFIG},)
   $(error Could not locate llvm-config, make sure it is installed and in your PATH, or set LLVM_CONFIG)
 else
@@ -118,15 +122,15 @@ libcrystal: $(LIB_CRYSTAL_TARGET)
 
 $(O)/all_spec: $(DEPS) $(SOURCES) $(SPEC_SOURCES)
 	@mkdir -p $(O)
-	$(BUILD_PATH) ./bin/crystal build $(FLAGS) $(SPEC_WARNINGS_OFF) -o $@ spec/all_spec.cr
+	$(EXPORT_CC) $(BUILD_PATH) ./bin/crystal build $(FLAGS) $(SPEC_WARNINGS_OFF) -o $@ spec/all_spec.cr
 
 $(O)/std_spec: $(DEPS) $(SOURCES) $(SPEC_SOURCES)
 	@mkdir -p $(O)
-	$(BUILD_PATH) ./bin/crystal build $(FLAGS) $(SPEC_WARNINGS_OFF) -o $@ spec/std_spec.cr
+	$(EXPORT_CC) $(BUILD_PATH) ./bin/crystal build $(FLAGS) $(SPEC_WARNINGS_OFF) -o $@ spec/std_spec.cr
 
 $(O)/compiler_spec: $(DEPS) $(SOURCES) $(SPEC_SOURCES)
 	@mkdir -p $(O)
-	$(BUILD_PATH) ./bin/crystal build $(FLAGS) $(SPEC_WARNINGS_OFF) -o $@ spec/compiler_spec.cr
+	$(EXPORT_CC) $(BUILD_PATH) ./bin/crystal build $(FLAGS) $(SPEC_WARNINGS_OFF) -o $@ spec/compiler_spec.cr
 
 $(O)/crystal: $(DEPS) $(SOURCES)
 	@mkdir -p $(O)