KVM: selftests: Avoid using forced target for generating arm64 headers

The 'prepare' target that generates the arm64 sysreg headers had no
prerequisites, so it wound up forcing a rebuild of all KVM selftests
each invocation. Add a rule for the generated headers and just have
dependents use that for a prerequisite.
Reported-by: Nina Schoetterl-Glausch <nsg@linux.ibm.com>
Fixes: 9697d84c ("KVM: selftests: Generate sysreg-defs.h and add to include path")
Tested-by: Nina Schoetterl-Glausch <nsg@linux.ibm.com>
Link: https://lore.kernel.org/r/20231027005439.3142015-3-oliver.upton@linux.devSigned-off-by: Oliver Upton <oliver.upton@linux.dev>

tools/testing/selftests/kvm/Makefile

@@ -22,10 +22,8 @@ arm64_tools_dir := $(top_srcdir)/tools/arch/arm64/tools/
 GEN_HDRS := $(top_srcdir)/tools/arch/arm64/include/generated/
 CFLAGS += -I$(GEN_HDRS)
 
-prepare:
+$(GEN_HDRS): $(wildcard $(arm64_tools_dir)/*)
 	$(MAKE) -C $(arm64_tools_dir)
-else
-prepare:
 endif
 
 LIBKVM += lib/assert.c
@@ -276,10 +274,10 @@ EXTRA_CLEAN += $(GEN_HDRS) \
 	       cscope.*
 
 x := $(shell mkdir -p $(sort $(dir $(LIBKVM_C_OBJ) $(LIBKVM_S_OBJ))))
-$(LIBKVM_C_OBJ): $(OUTPUT)/%.o: %.c prepare
+$(LIBKVM_C_OBJ): $(OUTPUT)/%.o: %.c $(GEN_HDRS)
 	$(CC) $(CFLAGS) $(CPPFLAGS) $(TARGET_ARCH) -c $< -o $@
 
-$(LIBKVM_S_OBJ): $(OUTPUT)/%.o: %.S prepare
+$(LIBKVM_S_OBJ): $(OUTPUT)/%.o: %.S $(GEN_HDRS)
 	$(CC) $(CFLAGS) $(CPPFLAGS) $(TARGET_ARCH) -c $< -o $@
 
 # Compile the string overrides as freestanding to prevent the compiler from
@@ -289,9 +287,10 @@ $(LIBKVM_STRING_OBJ): $(OUTPUT)/%.o: %.c
 	$(CC) $(CFLAGS) $(CPPFLAGS) $(TARGET_ARCH) -c -ffreestanding $< -o $@
 
 x := $(shell mkdir -p $(sort $(dir $(TEST_GEN_PROGS))))
+$(SPLIT_TESTS_OBJS): $(GEN_HDRS)
 $(TEST_GEN_PROGS): $(LIBKVM_OBJS)
 $(TEST_GEN_PROGS_EXTENDED): $(LIBKVM_OBJS)
-$(TEST_GEN_OBJ): prepare
+$(TEST_GEN_OBJ): $(GEN_HDRS)
 
 cscope: include_paths = $(LINUX_TOOL_INCLUDE) $(LINUX_HDR_PATH) include lib ..
 cscope: