[libc] fix -Wconversion (#77384)

Fixes the following from GCC:

    llvm-project/libc/src/string/memory_utils/op_x86.h:236:24: error:
conversion from ‘long unsigned int’ to ‘uint32_t’ {aka ‘unsigned int’}
may
    change value [-Werror=conversion]
      236 |   return (xored >> 32) | (xored & 0xFFFFFFFF);
          |          ~~~~~~~~~~~~~~^~~~~~~~~~~~~~~~~~~~~~

Link:
https://lab.llvm.org/buildbot/#/builders/250/builds/16236/steps/8/logs/stdio
Link: #74506

Author: nickdesaulniers

libc/src/string/memory_utils/op_x86.h

@@ -233,7 +233,8 @@ template <> LIBC_INLINE uint32_t neq<__m512i>(CPtr p1, CPtr p2, size_t offset) {
   const auto a = load<__m512i>(p1, offset);
   const auto b = load<__m512i>(p2, offset);
   const uint64_t xored = _mm512_cmpneq_epi8_mask(a, b);
-  return (xored >> 32) | (xored & 0xFFFFFFFF);
+  return static_cast<uint32_t>(xored >> 32) |
+         static_cast<uint32_t>(xored & 0xFFFFFFFF));
 }
 template <>
 LIBC_INLINE MemcmpReturnType cmp_neq<__m512i>(CPtr p1, CPtr p2, size_t offset) {