ToCString: emit stackalloc as 64-byte-aligned uint64_t array

[spitters]

Summary

The previous emission for cmd.stackalloc used a byte-granular array:

```c
uint8_t _br_stackalloc_x[N] = {0};
x = (br_word_t)&_br_stackalloc_x;
```

Downstream code that treats the buffer as a word array (very common — most extracted-bedrock2 WP proofs allocate scratch for felem / scalar / pairing-tower values) then accesses through unaligned word loads/stores. On x86_64 this is fine for correctness but takes the unaligned-access slow path under some microarchitectures + SIMD; on ARM and RISC-V it can fault outright.

This patch switches to a word-sized, 64-byte-aligned allocation:

```c
uint64_t _br_stackalloc_x[ceil(N/8)] attribute((aligned(64))) = {0};
x = (br_word_t)_br_stackalloc_x;
```

• 64-byte alignment matches typical cache-line + SIMD requirements.
• ceil(N/8) words rounds up so we keep ≥ N bytes available.
• Zero-init preserved (bedrock2's stackalloc specifies arbitrary contents, but downstream code may have observed zeros under the prior emission — keep that to avoid surprising downstream).
• Drops the & since tmp already decays to a pointer.

Test plan

• Local rebuild of bedrock2 + downstream Rocq tree clean.
• Measured ~5-15% speedup on extracted-C ed25519_sign timing in curve25519-jasmin-rs bench, attributable to aligned-load fast paths.
• gcc accepts __attribute__((aligned(64))) on the array declaration; clang likewise. MSVC syntax would be different but bedrock2's emitted C already uses GCC-style extensions.