ARROW-8166: [C++] fix AVX512 intrinsics fail with clang-8

__m512i_u undeclared in clang while _mm512_storeu_epi32 undefined in gcc,
using memcpy instead for the unaligned stroe.

BM_PlainDecodingBoolean with gcc get same level result with previous.

Signed-off-by: Frank Du <frank.du@intel.com>

Closes #6673 from jianxind/avx512-build-with-clang

Authored-by: Frank Du <frank.du@intel.com>
Signed-off-by: Wes McKinney <wesm+git@apache.org>

cpp/src/arrow/util/bpacking.h

@@ -39,20 +39,22 @@ namespace internal {
 #if defined(__AVX512F__)
 inline const uint32_t* unpack1_32(const uint32_t* in, uint32_t* out) {
   uint32_t inl = util::SafeLoad(in);
-  __m512i shifts, inls, masks;
+  __m512i shifts, inls, masks, result;
 
   inls = _mm512_set1_epi32(inl);
   masks = _mm512_set1_epi32(1);
 
   // shift the first 16 outs
   shifts = _mm512_set_epi32(15, 14, 13, 12, 11, 10, 9, 8, 7, 6, 5, 4, 3, 2, 1, 0);
-  *(__m512i_u*)out = _mm512_and_epi32(_mm512_srlv_epi32(inls, shifts), masks);
+  result = _mm512_and_epi32(_mm512_srlv_epi32(inls, shifts), masks);
+  memcpy(out, &result, 16 * sizeof(*out));
   out += 16;
 
   // shift the last 16 outs
   shifts =
       _mm512_set_epi32(31, 30, 29, 28, 27, 26, 25, 24, 23, 22, 21, 20, 19, 18, 17, 16);
-  *(__m512i_u*)out = _mm512_and_epi32(_mm512_srlv_epi32(inls, shifts), masks);
+  result = _mm512_and_epi32(_mm512_srlv_epi32(inls, shifts), masks);
+  memcpy(out, &result, 16 * sizeof(*out));
   out += 16;
 
   ++in;