tools: ifdtool: Write correct offset on 32-bit machine

On 32-bit machine strtol() returns LONG_MAX which is 0x7fffffff,
which is wrong for u-boot.rom components like u-boot-x86-16bit.bin.
Change to use strtoll() so that it works on both 32-bit and 64-bit
machines.

Reported-by: Fei Wang <wangfei.jimei@gmail.com>
Signed-off-by: Bin Meng <bmeng.cn@gmail.com>
Acked-by: Simon Glass <sjg@chromium.org>