tools/rv: Fix Makefile compiler options for clang

The following errors are showing up when compiling rv with clang:

 $ make HOSTCC=clang CC=clang LLVM_IAS=1
 [...]
  clang -O -g -DVERSION=\"6.8.0-rc1\" -flto=auto -ffat-lto-objects
  -fexceptions -fstack-protector-strong -fasynchronous-unwind-tables
  -fstack-clash-protection  -Wall -Werror=format-security
  -Wp,-D_FORTIFY_SOURCE=2 -Wp,-D_GLIBCXX_ASSERTIONS
  -Wno-maybe-uninitialized $(pkg-config --cflags libtracefs)
  -I include   -c -o src/utils.o src/utils.c
  clang: warning: optimization flag '-ffat-lto-objects' is not supported [-Wignored-optimization-argument]
  warning: unknown warning option '-Wno-maybe-uninitialized'; did you mean '-Wno-uninitialized'? [-Wunknown-warning-option]
  1 warning generated.

  clang -o rv -ggdb  src/in_kernel.o src/rv.o src/trace.o src/utils.o $(pkg-config --libs libtracefs)
  src/in_kernel.o: file not recognized: file format not recognized
  clang: error: linker command failed with exit code 1 (use -v to see invocation)
  make: *** [Makefile:110: rv] Error 1

Solve these issues by:
  - removing -ffat-lto-objects and -Wno-maybe-uninitialized if using clang
  - informing the linker about -flto=auto

Link: https://lkml.kernel.org/r/ed94a8ddc2ca8c8ef663cfb7ae9dd196c4a66b33.1707217097.git.bristot@kernel.org

Cc: stable@vger.kernel.org
Cc: Masami Hiramatsu <mhiramat@kernel.org>
Cc: Nathan Chancellor <nathan@kernel.org>
Cc: Nick Desaulniers <ndesaulniers@google.com>
Cc: Bill Wendling <morbo@google.com>
Cc: Justin Stitt <justinstitt@google.com>
Fixes: 4bc4b13 ("rv: Add rv tool")
Suggested-by: Donald Zickus <dzickus@redhat.com>
Signed-off-by: Daniel Bristot de Oliveira <bristot@kernel.org>

Author: Daniel Bristot de Oliveira

tools/verification/rv/Makefile

@@ -28,10 +28,15 @@ FOPTS	:=	-flto=auto -ffat-lto-objects -fexceptions -fstack-protector-strong \
 		-fasynchronous-unwind-tables -fstack-clash-protection
 WOPTS	:= 	-Wall -Werror=format-security -Wp,-D_FORTIFY_SOURCE=2 -Wp,-D_GLIBCXX_ASSERTIONS -Wno-maybe-uninitialized
 
+ifeq ($(CC),clang)
+  FOPTS := $(filter-out -ffat-lto-objects, $(FOPTS))
+  WOPTS := $(filter-out -Wno-maybe-uninitialized, $(WOPTS))
+endif
+
 TRACEFS_HEADERS	:= $$($(PKG_CONFIG) --cflags libtracefs)
 
 CFLAGS	:=	-O -g -DVERSION=\"$(VERSION)\" $(FOPTS) $(MOPTS) $(WOPTS) $(TRACEFS_HEADERS) $(EXTRA_CFLAGS) -I include
-LDFLAGS	:=	-ggdb $(EXTRA_LDFLAGS)
+LDFLAGS	:=	-flto=auto -ggdb $(EXTRA_LDFLAGS)
 LIBS	:=	$$($(PKG_CONFIG) --libs libtracefs)
 
 SRC	:=	$(wildcard src/*.c)