[v1.06][RISCV] Support for fetching mvendorid, marchid and mimpid (#286)

Getting these 3 RISC-V cpuinfo fields allows the detection of
microarchitecture (and other information), extending the RISC-V
detection capabilities. In particular, this is used here to detect
the marchid of Spacemit X60 uarch.

This commit also changes how the microarchitecture is fetched
(i.e., get_uarch) so that it does not rely only in the uarch field
in cpuinfo, but also on the marchid value.

src/common/cpu.h

@@ -25,6 +25,7 @@ enum {
   CPU_VENDOR_RISCV,
   CPU_VENDOR_SIFIVE,
   CPU_VENDOR_THEAD,
+  CPU_VENDOR_SPACEMIT,
 // OTHERS
   CPU_VENDOR_UNKNOWN,
   CPU_VENDOR_INVALID