objtool: Improve error message for bad file argument

If a nonexistent file is supplied to objtool, it complains with a
non-helpful error:

  open: No such file or directory

Improve it to:

  objtool: Can't open 'foo': No such file or directory
Reported-by: Markus <M4rkusXXL@web.de>
Signed-off-by: Josh Poimboeuf <jpoimboe@redhat.com>
Cc: Linus Torvalds <torvalds@linux-foundation.org>
Cc: Peter Zijlstra <peterz@infradead.org>
Cc: Thomas Gleixner <tglx@linutronix.de>
Link: http://lkml.kernel.org/r/406a3d00a21225eee2819844048e17f68523ccf6.1516025651.git.jpoimboe@redhat.comSigned-off-by: Ingo Molnar <mingo@kernel.org>

tools/objtool/elf.c

@@ -26,6 +26,7 @@
 #include <stdlib.h>
 #include <string.h>
 #include <unistd.h>
+#include <errno.h>
 
 #include "elf.h"
 #include "warn.h"
@@ -358,7 +359,8 @@ struct elf *elf_open(const char *name, int flags)
 
 	elf->fd = open(name, flags);
 	if (elf->fd == -1) {
-		perror("open");
+		fprintf(stderr, "objtool: Can't open '%s': %s\n",
+			name, strerror(errno));
 		goto err;
 	}