riscv: rework CPU identification [7/7]

[mhorne]

Summary

Change the reporting strategy to more closely follow what arm64
implements:

• Always print the one-line CPU summary when a core comes online
• Only print the additional fields (e.g. ISA) when they differ from the CPU before it

In the common case of identical CPUs this results in informative but
non-repetitive output.

[markj]

sys/riscv/riscv/identcpu.c

459

Indentation looks wrong here.