src/Makefile: log real/user/system time for build targets

For some of our build targets such as buildworld, buildkernel etc. we log
the real time in seconds how long the build runs. Now we log the user and
system time as well in milliseconds for most build targets.

Example output:

make buildworld
[....]

Time spent on target buildworld: real 2286.42 user 62870.84 sys 2294.22

PR: 287274