riscv64gcv.S -> riscv_rva23u64.S

With QEMU v8.2 the test command is now:

    for VLEN in 128 256 512 1024 ; do
        CC="$HOME/llvm-project/build/bin/clang" CARGO_TARGET_RISCV64GC_UNKNOWN_LINUX_GNU_LINKER=riscv64-linux-gnu-gcc CARGO_TARGET_RISCV64GC_UNKNOWN_LINUX_GNU_RUSTFLAGS="-L /usr/lib/gcc/riscv64-linux-gnu/12.2.0" CARGO_TARGET_RISCV64GC_UNKNOWN_LINUX_GNU_RUNNER="$HOME/qemu/build/qemu-riscv64 -L /usr/riscv64-linux-gnu -cpu rv64,v=true,vext_spec=v1.0,vlen=$VLEN,zbb=true,zvbb=true" cargo test --target riscv64gc-unknown-linux-gnu
    done

rust/guts/build.rs

@@ -232,7 +232,7 @@ fn build_neon_c_intrinsics() {
 fn build_riscv64gcv_assembly() {
     println!("cargo:rustc-cfg=blake3_riscv64gcv_ffi");
     let mut build = new_build();
-    let asm_path = "src/riscv64gcv.S";
+    let asm_path = "src/riscv_rva23u64.S";
     build.file(asm_path);
     build.flag("--target=riscv64");
     build.flag("-march=rv64gcv_zbb_zvbb1p0");

rust/guts/src/riscv64gcv.S → rust/guts/src/riscv_rva23u64.S

@@ -1,8 +1,11 @@
-// This implementation currently assumes riscv64gcv_zbb_zvbb. Zvbb in particular ("Vector
-// Bit-manipulation used in Cryptography") is a bleeding-edge extension that was only frozen a few
-// weeks ago at the time I'm writing this comment. Compiling and testing this code currently
-// requires quite a lot of effort, including building Clang from master and building QEMU from a
-// custom branch. Please don't expect this code to be usable on real hardware for some time.
+// This implementation targets the RVA23 profile, particularly V, Zvbb, and
+// Zbb, that is the vector extension and the bit-manipulation extensions. As of
+// December 2023, most real-world hardware does *not* support these extensions.
+// This implementation also assumes that misaligned vector loads and stores are
+// supported, in particular for the vlsseg8e32.v and vssseg8e32.v instructions.
+//
+// Compiling and testing this code requires very recent versions of Clang (v17)
+// and QEMU (v8.2).
 
 #define IV0 0x6A09E667
 #define IV1 0xBB67AE85