cmake: stop destroying intermediate link maps like zephyr_prebuilt.map

The different linker steps are all hardcoded to the output filename
"zephyr.map" and only the last one survives.

Un-harcode.

This removes some confusion when trying to follow who builds
what/when/how and it stops destroying intermediate linking information
useful when tracing/debugging new features or issues in the build.

Signed-off-by: Marc Herbert <marc.herbert@intel.com>