x86/vdso, build: When vdso2c fails, unlink the output

This avoids bizarre failures if make is run again.

Signed-off-by: Andy Lutomirski <luto@amacapital.net>
Link: http://lkml.kernel.org/r/1764385fe9931e8940b9d001132515448ea89523.1401464755.git.luto@amacapital.net


Signed-off-by: H. Peter Anvin <hpa@linux.intel.com>