Separate big-endian chunk ordering and add experimental note

- Clarify that little-endian chunk ordering is specified explicitly
- Move big-endian behavior to a separate NOTE section
- Document that big-endian ABI is experimental and may change

Author: kito-cheng

riscv-cc.adoc

@@ -668,9 +668,13 @@ arbitrary-sized integer type, where N is a postive integer larger than zero.
 `_BitInt(N)` types are stored in memory as follows:
 - For N ≤ 2×XLEN: Types follow the standard scalar endianness rules.
 - For N > 2×XLEN: Types are divided into 2×XLEN-bit chunks. Each chunk follows
-  the standard scalar endianness rules. Chunks are always stored in memory with
-  lower-addressed chunks containing less significant bits, regardless of the
-  system endianness.
+  the standard scalar endianness rules. On little-endian systems, chunks are
+  stored in memory with lower-addressed chunks containing less significant bits.
+
+NOTE: For big-endian systems with N > 2×XLEN, chunks are currently also stored
+with lower-addressed chunks containing less significant bits (same as little-endian).
+However, the big-endian ABI is still experimental and this behavior may change
+in the future.
 
 Unused bits of the `_BitInt(N)` are extended based on the signedness of the
 type when stored in memory or register. For signed `_BitInt(N)`, unused bits