The initial ROMs (program and linker) are already attached to the region
map of a forked process and don't need to be obtained again from an
external ROM service when the 'Child' class asks for them. For the program
image, the local Noux ROM service already returned an invalid dataspace
capability, but not for the linker.
Instead of adding another 'magic' ROM name for the local ROM service,
this commit implements an 'empty' ROM service, which returns an
invalid dataspace for the initial ROMs.
Fixes#2272
