hexagon: Add scalar DSP intrinsics

Add scalar (non-HVX) intrinsic wrappers for Hexagon DSP operations
covering arithmetic, multiply, shift, saturate, compare, and
floating-point instructions, mapping to llvm.hexagon.* LLVM builtins.

Includes the stdarch-gen-hexagon-scalar code generator crate which
produces scalar.rs from the LLVM hexagon_protos.h header file, and a
CI step to verify the generated output stays in sync.

Author: androm3da

.github/workflows/main.yml

@@ -331,6 +331,10 @@ jobs:
       run: |
         cargo run -p stdarch-gen-hexagon --release
         git diff --exit-code
+    # - name: Check hexagon scalar
+    #   run: |
+    #     cargo run -p stdarch-gen-hexagon-scalar --release
+    #     git diff --exit-code
 
   # Run some tests with Miri. Most stdarch functions use platform-specific intrinsics
   # that Miri does not support. Also Miri is reltively slow.