export.h: fix section name for CONFIG_TRIM_UNUSED_KSYMS for Clang

When enabling CONFIG_TRIM_UNUSED_KSYMS, the linker will warn about the
orphan sections:

(".discard.ksym") is being placed in '".discard.ksym"'

repeatedly when linking vmlinux.  This is because the stringification
operator, `#`, in the preprocessor escapes strings.  GCC and Clang differ
in how they treat section names that contain \".

The portable solution is to not use a string literal with the preprocessor
stringification operator.

Fixes: commit bbda5ec6 ("kbuild: simplify dependency generation for CONFIG_TRIM_UNUSED_KSYMS")
Reported-by: kbuild test robot <lkp@intel.com>
Suggested-by: Kees Cook <keescook@chromium.org>
Signed-off-by: Nick Desaulniers <ndesaulniers@google.com>
Signed-off-by: Andrew Morton <akpm@linux-foundation.org>
Reviewed-by: Kees Cook <keescook@chromium.org>
Cc: Nathan Chancellor <natechancellor@gmail.com>
Cc: Masahiro Yamada <yamada.masahiro@socionext.com>
Cc: Matthias Maennich <maennich@google.com>
Cc: Jessica Yu <jeyu@kernel.org>
Cc: Greg Kroah-Hartman <gregkh@linuxfoundation.org>
Cc: Will Deacon <will@kernel.org>
Link: https://bugs.llvm.org/show_bug.cgi?id=42950
Link: https://github.com/ClangBuiltLinux/linux/issues/1166
Link: https://lkml.kernel.org/r/20200929190701.398762-1-ndesaulniers@google.comSigned-off-by: Linus Torvalds <torvalds@linux-foundation.org>

include/linux/export.h

@@ -130,7 +130,7 @@ struct kernel_symbol {
  * discarded in the final link stage.
  */
 #define __ksym_marker(sym)	\
-	static int __ksym_marker_##sym[0] __section(".discard.ksym") __used
+	static int __ksym_marker_##sym[0] __section(.discard.ksym) __used
 
 #define __EXPORT_SYMBOL(sym, sec, ns)					\
 	__ksym_marker(sym);						\