[flang] Enable RISC-V for x86CompatibleBehavior in floating point flag

Fixes #59132 by mitigating the behavior of not setting underflow flag on RISC-V platform.

Reviewed By: vzakhari

Differential Revision: https://reviews.llvm.org/D138503

Author: realqhc

flang/include/flang/Evaluate/target.h

@@ -25,7 +25,7 @@ struct Rounding {
   // (viz., fail to set the Underflow flag when an inexact product of a
   // multiplication is rounded up to a normal number from a subnormal
   // in some rounding modes)
-#if __x86_64__
+#if __x86_64__ || __riscv
   bool x86CompatibleBehavior{true};
 #else
   bool x86CompatibleBehavior{false};