objtool: Fix verbose disassembly if CROSS_COMPILE isn't set

In verbose mode, when printing the disassembly of affected functions, if
CROSS_COMPILE isn't set, the objdump command string gets prefixed with
"(null)".

Somehow this worked before.  Maybe some versions of glibc return an
empty string instead of NULL.  Fix it regardless.

[ jpoimboe: Rewrite commit log. ]

Fixes: ca65346 ("objtool: Add verbose option for disassembling affected functions")
Signed-off-by: David Laight <david.laight.linux@gmail.com>
Signed-off-by: Josh Poimboeuf <jpoimboe@kernel.org>
Signed-off-by: Ingo Molnar <mingo@kernel.org>
Cc: Linus Torvalds <torvalds@linux-foundation.org>
Link: https://lore.kernel.org/r/20250215142321.14081-1-david.laight.linux@gmail.com
Link: https://lore.kernel.org/r/b931a4786bc0127aa4c94e8b35ed617dcbd3d3da.1743481539.git.jpoimboe@kernel.org

Author: David Laight

tools/objtool/check.c

@@ -4523,6 +4523,8 @@ static void disas_funcs(const char *funcs)
 	char *cmd;
 
 	cross_compile = getenv("CROSS_COMPILE");
+	if (!cross_compile)
+		cross_compile = "";
 
 	objdump_str = "%sobjdump -wdr %s | gawk -M -v _funcs='%s' '"
 			"BEGIN { split(_funcs, funcs); }"