[RISCV] Add groupid/bitmask for RISC-V extension

Base on riscv-non-isa/riscv-c-api-doc#74.

This patch defines the groupid/bitmask in RISCVFeatures.td and generates the corresponding table in RISCVTargetParserDef.inc.

The groupid/bitmask of extensions provides an abstraction layer between the compiler and runtime functions.

Author: BeMg

llvm/include/llvm/TargetParser/RISCVTargetParser.h

@@ -24,6 +24,14 @@ class Triple;
 
 namespace RISCV {
 
+namespace RISCVExtensionBitmaskTable {
+struct RISCVExtensionBitmask {
+  const char *Name;
+  unsigned GroupID;
+  uint64_t Bitmask;
+};
+} // namespace RISCVExtensionBitmaskTable
+
 // We use 64 bits as the known part in the scalable vector types.
 static constexpr unsigned RVVBitsPerBlock = 64;