[RISCV] Add rv32 or rv64 suffix to generic cpu name

[StephenFan]

Summary

Currently, host cpu name is determined by uarch line which is read from
/proc/cpuinfo. But since qemu's cpuinfo doesn't have an uarch line,
getHostCPUNameForRISCV would just return "generic".

This patch adds rv32 or rv64 suffix to "generic" string to avoid issues
like can't recognize "generic" cpu name.

[kito-cheng]

We have generic in RISCV.td since D121149, is it not work for this case?

[StephenFan]

Determine if it is rv32 or rv64 by __riscv_xlen

[StephenFan]

In D131851#3722311, @kito-cheng wrote:

We have generic in RISCV.td since D121149, is it not work for this case?

Oh, thanks for reminding me. Let me see.

llvm/lib/Support/Host.cpp

if (Lines[I].startswith("uarch")) {
       UArch = Lines[I].substr(5).ltrim("\t :");
       break;
     }
   }
 
   return StringSwitch<const char *>(UArch)
       .Case("sifive,u74-mc", "sifive-u74")
       .Case("sifive,bullet0", "sifive-u74")
+#if __riscv_xlen == 64
+      .Default("generic-rv64");
+#elif __riscv_xlen == 32
+      .Default("generic-rv32");
+#else
       .Default("generic");
+#endif
 }
 
 StringRef sys::detail::getHostCPUNameForBPF() {
 #if !defined(__linux__) || !defined(__x86_64__)
   return "generic";
 #else
   uint8_t v3_insns[40] __attribute__ ((aligned (8))) =
       /* BPF_MOV64_IMM(BPF_REG_0, 0) */