Add -flax-vector-conversions for PowerPC AltiVec/VSX HACL build

GCC with -std=c11 and -maltivec treats 'bool' as '__vector __bool int'
in certain struct access patterns, causing type errors in the HACL*
BLAKE2 SIMD128 code. Adding -flax-vector-conversions resolves this
without affecting code generation.

Verified on IBM POWER8 S824 with GCC 10.5 — HACL Blake2s_Simd128.c
now compiles cleanly with this flag.

Co-Authored-By: Claude Opus 4.6 (1M context) <noreply@anthropic.com>

Author: Scottcjn

configure

@@ -8133,7 +8133,7 @@ then
     dnl x86 SSE not available; check for PowerPC AltiVec/VSX (Power8+).
     dnl HACL* libintvector.h has a complete vec128 implementation for __powerpc64__.
     AX_CHECK_COMPILE_FLAG([-maltivec -mvsx],[
-      [LIBHACL_SIMD128_FLAGS="-maltivec -mvsx"]
+      [LIBHACL_SIMD128_FLAGS="-maltivec -mvsx -flax-vector-conversions"]
 
       AC_DEFINE([_Py_HACL_CAN_COMPILE_VEC128], [1], [
         HACL* library can compile SIMD128 implementations])