Adding special rule for _BitInt(32) on RV64

Author: kito-cheng

riscv-cc.adoc

@@ -189,6 +189,12 @@ When passed in registers or on the stack, `_BitInt(N)` types are handled as
 follows:
 - For N < XLEN: Types are widened to XLEN bits, with upper bits sign-extended
   for signed `_BitInt(N)` or zero-extended for unsigned `_BitInt(N)`.
+  However, for N = 32 in RV64, both signed and unsigned types are sign-extended.
++
+[NOTE]
+====
+This ensures that `_BitInt(32)` behaves consistently with regular `int` types in RV64.
+====
 - For N = XLEN: Types are passed using the same rules as scalars that are XLEN
   bits wide.
 - For XLEN < N ≤ 2×XLEN: Types are passed using the same rules as scalars that