selftests/fchmodat2: fix clang build failure due to -static-libasan

gcc requires -static-libasan in order to ensure that Address Sanitizer's
library is the first one loaded. However, this leads to build failures
on clang, when building via:

    make LLVM=1 -C tools/testing/selftests

However, clang already does the right thing by default: it statically
links the Address Sanitizer if -fsanitize is specified. Therefore,
simply omit -static-libasan for clang builds. And leave behind a
comment, because the whole reason for static linking might not be
obvious.

Cc: Ryan Roberts <ryan.roberts@arm.com>
Signed-off-by: John Hubbard <jhubbard@nvidia.com>
Reviewed-by: Nathan Chancellor <nathan@kernel.org>
Signed-off-by: Shuah Khan <skhan@linuxfoundation.org>

tools/testing/selftests/fchmodat2/Makefile

 # SPDX-License-Identifier: GPL-2.0-or-later
 
-CFLAGS += -Wall -O2 -g -fsanitize=address -fsanitize=undefined -static-libasan $(KHDR_INCLUDES)
+CFLAGS += -Wall -O2 -g -fsanitize=address -fsanitize=undefined $(KHDR_INCLUDES)
+
+# gcc requires -static-libasan in order to ensure that Address Sanitizer's
+# library is the first one loaded. However, clang already statically links the
+# Address Sanitizer if -fsanitize is specified. Therefore, simply omit
+# -static-libasan for clang builds.
+ifeq ($(LLVM),)
+    CFLAGS += -static-libasan
+endif
+
 TEST_GEN_PROGS := fchmodat2_test
 
 include ../lib.mk