objtool: Don't fail the kernel build on fatal errors

This is basically a revert of commit 644592d ("objtool: Fail the
kernel build on fatal errors").

That change turned out to be more trouble than it's worth.  Failing the
build is an extreme measure which sometimes gets too much attention and
blocks CI build testing.

These fatal-type warnings aren't yet as rare as we'd hope, due to the
ever-increasing matrix of supported toolchains/plugins and their
fast-changing nature as of late.

Also, there are more people (and bots) looking for objtool warnings than
ever before, so even non-fatal warnings aren't likely to be ignored for
long.

Suggested-by: Nick Desaulniers <ndesaulniers@google.com>
Reviewed-by: Miroslav Benes <mbenes@suse.cz>
Reviewed-by: Nick Desaulniers <ndesaulniers@google.com>
Reviewed-by: Kamalesh Babulal <kamalesh@linux.vnet.ibm.com>
Signed-off-by: Josh Poimboeuf <jpoimboe@redhat.com>

tools/objtool/check.c

@@ -2928,14 +2928,10 @@ int check(struct objtool_file *file)
 	warnings += ret;
 
 out:
-	if (ret < 0) {
-		/*
-		 *  Fatal error.  The binary is corrupt or otherwise broken in
-		 *  some way, or objtool itself is broken.  Fail the kernel
-		 *  build.
-		 */
-		return ret;
-	}
-
+	/*
+	 *  For now, don't fail the kernel build on fatal warnings.  These
+	 *  errors are still fairly common due to the growing matrix of
+	 *  supported toolchains and their recent pace of change.
+	 */
 	return 0;
 }