Cross-compilation fixes (x86/64 on i386).

Cross-compilation fixes (x86/64 on i386).

Use strtoull to parse the final_exec_addr value from the command
line. Whe cross compiling on i386 for x86-64 unsigned long is 32 bit,
which leads to final_exec_addr being truncated to 32 bits by strtoul,
which leads to grub refusing to load the resulting image. Please
apply.

Use the target nm(1) when cross compiling to pick the _end address
from the Xen binary.

Signed-Off-By: Muli Ben-Yehuda <mulix@mulix.org>