Add RISCV vector extension (#289)

Co-authored-by: Michael Roe <michael-roe@users.noreply.github.com>

include/internal/hwcaps.h

@@ -214,6 +214,7 @@ CPU_FEATURES_START_CPP_NAMESPACE
 #define RISCV_HWCAP_D (1UL << ('D' - 'A'))
 #define RISCV_HWCAP_Q (1UL << ('Q' - 'A'))
 #define RISCV_HWCAP_C (1UL << ('C' - 'A'))
+#define RISCV_HWCAP_V (1UL << ('V' - 'A'))
 
 typedef struct {
   unsigned long hwcaps;