tools: compiler-gcc.h: Guard error attribute use with __has_attribute

When building objtool with HOSTCC=clang, there are several errors along
the lines of

  orc_dump.c:201:28: error: unknown attribute 'error' ignored [-Werror,-Wunknown-attributes]

This occurs after commit 4e59869aa6 ("compiler-gcc.h: drop checks for
older GCC versions"), which removed the GCC_VERSION gating.  The removed
version check just so happened to prevent __compiletime_error() from
being defined with clang because it pretends to be GCC 4.2.1 for
compatibility but the error attribute was not added to clang until
14.0.0.

Commit 815f0ddb34 ("include/linux/compiler*.h: make compiler-*.h
mutually exclusive") and commit a3f8a30f3f ("Compiler Attributes: use
feature checks instead of version checks") refactored the handling of
attributes in the main kernel to avoid situations like this but that
refactoring has never been done for the tools directory.

Refactoring is a rather large undertaking and this has never been an
issue before so instead, just guard the definition of
__compiletime_error() with __has_attribute() so that there are no more
errors.

Fixes: 4e59869aa6 ("compiler-gcc.h: drop checks for older GCC versions")
Signed-off-by: Nathan Chancellor <nathan@kernel.org>
Signed-off-by: Linus Torvalds <torvalds@linux-foundation.org>

tools/include/linux/compiler-gcc.h

@@ -16,7 +16,9 @@
 # define __fallthrough __attribute__ ((fallthrough))
 #endif
 
-#define __compiletime_error(message) __attribute__((error(message)))
+#if __has_attribute(__error__)
+# define __compiletime_error(message) __attribute__((error(message)))
+#endif
 
 /* &a[0] degrades to a pointer: a different type from an array */
 #define __must_be_array(a)	BUILD_BUG_ON_ZERO(__same_type((a), &(a)[0]))