Build of release.x64 file unnecessarily recreates generated header files

[nyh]

When I run

make build/release.x64/core/power.o

After this object file has already been created, I get the following unnecessary activity:

$ make build/release.x64/core/power.o
Building into build/release.x64
  GEN gen/include/osv/version.h
  GEN gen/include/osv/drivers_config.h

When I don't add the ".x64", the correct thing - i.e., absolutely nothing, happens:

$ make build/release/core/power.o 
Building into build/release.x64
make: Nothing to be done for 'build/release/core/power.o'.

This is not a serious problem, of course, but it was strange enough that I wanted to open the issue and perhaps later solve it.