Revert asm_offset CMake code to work with 3.10

Revert asm_offset CMake code to work with 3.10

The revertion of gen_defines.asm generation during cross compilation changed the
form of add_custom_command used, which inadvertently relied on CMake 3.16 support.

From the CMake Docs:
DEPENDS:
...
A target-level dependency is added if any dependency is a byproduct of a target or
any of its build events in the same directory to ensure the byproducts will be
available before this target is built.