hexagon: support for OP_CPY, host buffers now optional, hvx-utils refactoring and optimizations (#18822)

* hexagon: disable repack buffers if host buffers are disabled, improved handling of env vars

* hexagon: add support for OP_CPY fp16/fp32 -> fp16/fp32

Factore out all hvx_copy functions into hvx-copy.h header and reduced code duplication.
Update HTP ops infra to support OP_CPY

* hexagon: cleanup and refactor hex/hvx/htp headers and helper libs

hex is basically all scalar/core platform stuff (L2, DMA, basic utils)
hvx is all hvx related utils, helpers, etc
htp is higher level stuff like Ops, etc

hvx-utils library got a nice round of cleanup and refactoring to reduce duplication

use hvx_vec_store_a where possible

* hexagon: refactor HVX sigmoid functions to hvx-sigmoid.h

Moved sigmoid and tanh vector functions from hvx-utils.h to a new header
hvx-sigmoid.h. Implemented aligned and unaligned variants for sigmoid
array processing using a macro pattern similar to hvx-copy.h. Updated
act-ops.c to use the new aligned variant hvx_sigmoid_f32_aa. Removed
unused hvx-sigmoid.c.

* hexagon: factor out hvx-sqrt.h

* hexagon: mintor update to hvx-utils.h

* hexagon: remove spurios log

* hexagon: factor out and optimize hvx_add/sub/mul

* hexagon: remove _opt variants of add/sub/mul as they simply fully aligned versions

* hexagon: refactor reduction functions to hvx-reduce.h

Moved `hvx_self_max_f32` and `hvx_self_sum_f32` from `hvx-utils.h`/`.c` to `hvx-reduce.h`.
Renamed them to `hvx_reduce_max_f32` and `hvx_reduce_sum_f32`.
Added aligned (`_a`) and unaligned (`_u`) variants and used macros to unify logic.
Updated `softmax-ops.c` to use the new functions.

* hexagon: refactor the rest of arithmetic functions to hvx-arith.h

Moved `hvx_sum_of_squares_f32`, `hvx_min_scalar_f32`, and `hvx_clamp_scalar_f32` from `hvx-utils.c/h` to `hvx-arith.h`. Implemented aligned/unaligned variants (`_aa`, `_au`, etc.) and used macros to reduce code duplication. Updated `hvx_min_scalar_f32` and `hvx_clamp_scalar_f32` to use `dst, src, ..., n` argument order. Updated call sites in `act-ops.c`.

Refactor Hexagon HVX arithmetic functions (min, clamp) to hvx-arith.h

Moved `hvx_min_scalar_f32` and `hvx_clamp_scalar_f32` from `hvx-utils.c/h` to `hvx-arith.h`. Implemented aligned/unaligned variants (`_aa`, `_au`, etc.) and used macros to reduce code duplication. Updated these functions to use `dst, src, ..., n` argument order and updated call sites in `act-ops.c`. `hvx_sum_of_squares_f32` remains in `hvx-utils.c` as requested.

* hexagon: refactor hvx_sum_of_squares_f32

- Modify `hvx_sum_of_squares_f32` in `ggml/src/ggml-hexagon/htp/hvx-reduce.h` to use `dst, src` signature.
- Implement `_a` (aligned) and `_u` (unaligned) variants for `hvx_sum_of_squares_f32`.
- Update `hvx_reduce_loop_body` macro to support both returning and storing results via `finalize_op`.
- Update existing reduction functions in `hvx-reduce.h` to use the updated macro.
- Update `rms_norm_htp_f32` in `ggml/src/ggml-hexagon/htp/unary-ops.c` to match the new signature.

* hexagon: use hvx_splat instead of memset

* hexagon: consistent use of f32/f16 in all function names to match the rest of GGML

* hexagon: fix hvx_copy_f16_f32 on v75 and older

* hexagon: update readme to include GGML_HEXAGON_EXPERIMENTAL

* scripts: update snapdragon/adb scripts to enable host param

ggml/src/ggml-hexagon/htp/hex-fastdiv.h

@@ -0,0 +1,37 @@
+#ifndef HEX_FASTDIV_H
+#define HEX_FASTDIV_H
+
+// See https://gmplib.org/~tege/divcnst-pldi94.pdf figure 4.1.
+// Precompute mp (m' in the paper) and L such that division
+// can be computed using a multiply (high 32b of 64b result)
+// and a shift:
+//
+// n/d = (mulhi(n, mp) + n) >> L;
+struct fastdiv_values {
+    uint32_t mp;
+    uint32_t l;
+};
+
+static inline struct fastdiv_values init_fastdiv_values(uint32_t d) {
+    struct fastdiv_values result = { 0, 0 };
+    // compute L = ceil(log2(d));
+    while (result.l < 32 && ((uint32_t) 1 << result.l) < d) {
+        ++(result.l);
+    }
+
+    result.mp = (uint32_t) (((uint64_t) 1 << 32) * (((uint64_t) 1 << result.l) - d) / d + 1);
+    return result;
+}
+
+static inline uint32_t fastdiv(uint32_t n, const struct fastdiv_values * vals) {
+    // Compute high 32 bits of n * mp
+    const uint32_t hi = (uint32_t) (((uint64_t) n * vals->mp) >> 32);  // mulhi(n, mp)
+    // add n, apply bit shift
+    return (hi + n) >> vals->l;
+}
+
+static inline uint32_t fastmodulo(uint32_t n, uint32_t d, const struct fastdiv_values * vals) {
+    return n - fastdiv(n, vals) * d;
+}
+
+#endif /* HEX_FASTDIV_H */