selftests/x86: fix Makefile dependencies to work with clang

When building with clang, via:

    make LLVM=1 -C tools/testing/selftests

...the following build failure occurs in selftests/x86:

   clang: error: cannot specify -o when generating multiple output files

This happens because, although gcc doesn't complain if you invoke it
like this:

    gcc file1.c header2.h

...clang won't accept that form--it rejects the .h file(s). Also, the
above approach is inaccurate anyway, because file.c includes header2.h
in this case, and the inclusion of header2.h on the invocation is an
artifact of the Makefile's desire to maintain dependencies.

In Makefiles of this type, a better way to do it is to use Makefile
dependencies to trigger the appropriate incremental rebuilds, and
separately use file lists (see EXTRA_FILES in this commit) to track what
to pass to the compiler.

This commit splits those concepts up, by setting up both EXTRA_FILES and
the Makefile dependencies with a single call to the new Makefile
function extra-files.

That fixes the build failure, while still providing the correct
dependencies in all cases.
Acked-by: Muhammad Usama Anjum <usama.anjum@collabora.com>
Signed-off-by: John Hubbard <jhubbard@nvidia.com>
Signed-off-by: Shuah Khan <skhan@linuxfoundation.org>

tools/testing/selftests/x86/Makefile

@@ -73,10 +73,10 @@ all_64: $(BINARIES_64)
 EXTRA_CLEAN := $(BINARIES_32) $(BINARIES_64)
 
 $(BINARIES_32): $(OUTPUT)/%_32: %.c helpers.h
-	$(CC) -m32 -o $@ $(CFLAGS) $(EXTRA_CFLAGS) $^ -lrt -ldl -lm
+	$(CC) -m32 -o $@ $(CFLAGS) $(EXTRA_CFLAGS) $< $(EXTRA_FILES) -lrt -ldl -lm
 
 $(BINARIES_64): $(OUTPUT)/%_64: %.c helpers.h
-	$(CC) -m64 -o $@ $(CFLAGS) $(EXTRA_CFLAGS) $^ -lrt -ldl
+	$(CC) -m64 -o $@ $(CFLAGS) $(EXTRA_CFLAGS) $< $(EXTRA_FILES) -lrt -ldl
 
 # x86_64 users should be encouraged to install 32-bit libraries
 ifeq ($(CAN_BUILD_I386)$(CAN_BUILD_X86_64),01)
@@ -100,10 +100,19 @@ warn_32bit_failure:
 	exit 0;
 endif
 
-# Some tests have additional dependencies.
-$(OUTPUT)/sysret_ss_attrs_64: thunks.S
-$(OUTPUT)/ptrace_syscall_32: raw_syscall_helper_32.S
-$(OUTPUT)/test_syscall_vdso_32: thunks_32.S
+# Add an additional file to the source file list for a given target, and also
+# add a Makefile dependency on that same file. However, do these separately, so
+# that the compiler invocation ("$(CC) file1.c file2.S") is not combined with
+# the dependencies ("header3.h"), because clang, unlike gcc, will not accept
+# header files as an input to the compiler invocation.
+define extra-files
+$(OUTPUT)/$(1): EXTRA_FILES := $(2)
+$(OUTPUT)/$(1): $(2)
+endef
+
+$(eval $(call extra-files,sysret_ss_attrs_64,thunks.S))
+$(eval $(call extra-files,ptrace_syscall_32,raw_syscall_helper_32.S))
+$(eval $(call extra-files,test_syscall_vdso_32,thunks_32.S))
 
 # check_initial_reg_state is special: it needs a custom entry, and it
 # needs to be static so that its interpreter doesn't destroy its initial