perf tools riscv: Add support for get_cpuid_str function

perf tools riscv: Add support for get_cpuid_str function

The get_cpuid_str function returns the string that contains values of
MVENDORID, MARCHID and MIMPID in hex format separated by coma.

The values themselves are taken from first cpu entry in "/proc/cpuid"
that contains "mvendorid", "marchid" and "mimpid".

Signed-off-by: Nikita Shubin <n.shubin@yadro.com>
Tested-by: Kautuk Consul <kconsul@ventanamicro.com>
Acked-by: Palmer Dabbelt <palmer@rivosinc.com>
Cc: Albert Ou <aou@eecs.berkeley.edu>
Cc: Alexander Shishkin <alexander.shishkin@linux.intel.com>
Cc: Anup Patel <anup@brainfault.org>
Cc: Ingo Molnar <mingo@redhat.com>
Cc: Jiri Olsa <jolsa@kernel.org>
Cc: Mark Rutland <mark.rutland@arm.com>
Cc: Namhyung Kim <namhyung@kernel.org>
Cc: Palmer Dabbelt <palmer@dabbelt.com>
Cc: Paul Walmsley <paul.walmsley@sifive.com>
Cc: Peter Zijlstra <peterz@infradead.org>
Cc: linux-riscv@lists.infradead.org
Cc: linux@yadro.com
Link: https://lore.kernel.org/r/20220815132251.25702-2-nikita.shubin@maquefel.me
Signed-off-by: Arnaldo Carvalho de Melo <acme@redhat.com>