Seems like horizon has basic RISC-V support, e.g. the valid_arches array
in meta.cc already contains riscv. However, it seems the my_arch
function was not adjusted for riscv yet since the software likekly
wasn't compiled on riscv before. By adding the neccessary
riscv-ifdef-dance to my_arch from meta.cc it compiles fine on riscv.

This patch assumes that riscv means RV32 and riscv64 means RV64.

See also https://github.com/riscv/riscv-c-api-doc/blob/master/riscv-c-api.md

diff -upr horizon-0.9.6.orig/hscript/meta.cc horizon-0.9.6/hscript/meta.cc
--- horizon-0.9.6.orig/hscript/meta.cc	2021-07-10 19:29:55.073675786 +0200
+++ horizon-0.9.6/hscript/meta.cc	2021-07-10 19:43:31.674242939 +0200
@@ -716,6 +716,10 @@ const std::string my_arch(const Horizon:
         return "pmmx";
 #   elif defined(__x86_64__)
         return "x86_64";
+#   elif defined(__riscv) && __riscv_xlen == 32
+        return "riscv";
+#   elif defined(__riscv) && __riscv_xlen == 64
+        return "riscv64";
 #   elif defined(__mips64)
 #       if defined(__BYTE_ORDER__) && __BYTE_ORDER__ == __ORDER_LITTLE_ENDIAN__
         return "mips64el";