riscv: rework CPU identification [5/6]

[mhorne]

Summary

Print the standard (single-letter) ISA extensions. Limit the hwcap stuff
to its own function, and track the extension bits in struct cpu_desc. An
int is used for compatibility with the %b printf modifier, and because
we know the single-letter ISA bits will never go above 26.

[mhorne]

Rebase.

[markj]

sys/riscv/riscv/identcpu.c

261

Extra blank line.

[mhorne]

Rebase. This patch becomes much simpler.