On LP64 RISC-V int64_t is long, which collides with the explicit long
overloads in sc_bit.h.  Use long long for sc_dt::int64 there, as later
SystemC releases do unconditionally.

https://bugs.gentoo.org/926545
--- a/src/sysc/datatypes/int/sc_nbdefs.h
+++ b/src/sysc/datatypes/int/sc_nbdefs.h
@@ -177,7 +177,7 @@
 // Support for the long long type. This type is not in the standard
 // but is usually supported by compilers.
 #ifndef _WIN32
-#   if defined(__x86_64__) || defined(__aarch64__)
+#   if defined(__x86_64__) || defined(__aarch64__) || defined(__riscv)
         typedef long long          int64;
         typedef unsigned long long uint64;
 #   else