[PATCH 1/2] perf tools riscv: Allow get_cpuid return empty MARCH and MIMP

[Inochi Amaoto]

> > The T-HEAD C9xx series CPU only has MVENDOR defined, and left MARCH
> > and MIMP unimplemented.
>
> According to the docs you can still read them, but it's hardwired to
> 64h0.
>
> How it's supposed to distinguish c906 and c910 for example ?

It is unnecessary to distinguish c9xx, their event index is compatible.
The dtb and opensbi will final decide which event can be used.

> What does /proc/cpuinfo shows on c9xx ? Why can't we use zeroes ?

The content is as follows.

processor     : 0
hart          : 0
isa           : rv64imafdc
mmu           : sv39
uarch         : thead,c910
mvendorid     : 0x5b7
marchid       : 0x0
mimpid        : 0x0

The `mvendorid`, `marchid`, `mimpid` are the same across allwinner D1 (C906),
T-HEAD th1520 (C910) and the sophgo mango (C920). It seems T-HEAD use MCPUID
CSR to store CPU info. But this is not standard and not shown in cpuinfo.