https://github.com/riscv/riscv-c-api-doc/blob/master/riscv-c-api.md#preprocessor-definitions

diff -upr a/LibSupport.h b/LibSupport.h
--- a/LibSupport.h	2021-01-15 22:45:18.000000000 +0100
+++ b/LibSupport.h	2021-09-01 20:06:16.118220400 +0200
@@ -78,6 +78,8 @@
     || defined(_M_ARM) || defined(__arm__) || defined(__thumb__)	\
     || defined(__powerpc__) || defined(__PPC__)
 #       define RADIX_BITS                      32
+#   elif defined(__riscv)
+#       define RADIX_BITS                      __riscv_xlen
 #   else
 #       error Unable to determine RADIX_BITS from compiler environment
 #   endif