sse2_64_32.h: Fix compile errors under 32bit GCC

[Alcaro]

In file included from .../src/libsodium/crypto_stream/chacha20/dolbeau/chacha20_dolbeau-ssse3.c:9:
.../src/libsodium/include/sodium/private/sse2_64_32.h: In function 'sodium__mm_set_epi64x':
.../src/libsodium/include/sodium/private/sse2_64_32.h:21:1: warning: SSE vector return without SSE enabled changes the ABI [-Wpsabi]
 {
 ^
In file included from .../src/libsodium/include/sodium/private/sse2_64_32.h:14,
                 from .../src/libsodium/crypto_stream/chacha20/dolbeau/chacha20_dolbeau-ssse3.c:9:
.../i686-w64-mingw32/8.1.0/include/emmintrin.h:601:1: error: inlining failed in call to always_inline '_mm_set_epi32': target specific option mismatch
 _mm_set_epi32 (int __q3, int __q2, int __q1, int __q0)
 ^~~~~~~~~~~~~
In file included from .../src/libsodium/crypto_stream/chacha20/dolbeau/chacha20_dolbeau-ssse3.c:9:
.../src/libsodium/include/sodium/private/sse2_64_32.h:24:12: note: called from here
     return _mm_set_epi32(x1.as32[1], x1.as32[0], x0.as32[1], x0.as32[0]);
            ^~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~

pragma GCC push_options was added in GCC 4.4, same as #pragma GCC target (source), so this doesn't raise compiler requirements.

[jedisct1]

Thanks!

Actually it doesn't make sense to include this where SSE2 code is not expected.

So, instead of including it with general headers, it's been moved closer to other Intel intrinsics related headers.