Fix SSE build failure with MSVC

[cyb70289]

MSVC does not define __SSEn__ macros. Currently, we have to specify /arch:AVXn for an SSE only binary.

Fix #561

[cyb70289]

@serge-sans-paille @JohanMabille
I don't have windows dev env for a local test. Would you try if this PR is okay? Thanks.

[serge-sans-paille]

I'm puzzled: I activated SSE2 testing here: #563 but it doesn't show any error

[cyb70289]

Reproduced this error with below trivial code.

#include <xsimd/xsimd.hpp>

int main(void) {
  xsimd::batch<int8_t, xsimd::sse4_2> batch;
  return 0;
}

Tested on windows-10, visual studio community 2019,
command: cl -I include /arch:SSE2 test.cpp, same error without /arch. OK if /arch:AVX.

Logs:

λ cl -I include /arch:SSE2 test.cpp
Microsoft (R) C/C++ Optimizing Compiler Version 19.25.28612 for x64
Copyright (C) Microsoft Corporation.  All rights reserved.

cl : Command line warning D9002 : ignoring unknown option '/arch:SSE2'
test.cpp
C:\tools\cmder\xsimd\include\xsimd\memory\xsimd_aligned_allocator.hpp(40): error C2039: 'alignment': is not a member of 'xsimd::unavailable'
C:\tools\cmder\xsimd\include\xsimd\types\../config/xsimd_arch.hpp(57): note: see declaration of 'xsimd::unavailable'
C:\tools\cmder\xsimd\include\xsimd\memory\xsimd_aligned_allocator.hpp(40): error C3861: 'alignment': identifier not found
C:\tools\cmder\xsimd\include\xsimd\types\../memory/xsimd_alignment.hpp(46): error C2976: 'xsimd::aligned_allocator': too few template arguments
C:\tools\cmder\xsimd\include\xsimd\memory\xsimd_aligned_allocator.hpp(42): note: see declaration of 'xsimd::aligned_allocator'
C:\tools\cmder\xsimd\include\xsimd\types\../memory/xsimd_alignment.hpp(46): error C2975: 'Align': invalid template argument for 'xsimd::aligned_allocator', expected compile-time constant expression
C:\tools\cmder\xsimd\include\xsimd\memory\xsimd_aligned_allocator.hpp(40): note: see declaration of 'Align'
C:\tools\cmder\xsimd\include\xsimd\types\./xsimd_batch.hpp(23): error C2504: 'xsimd::types::simd_register<T,A>': base class undefined
        with
        [
            T=int8_t,
            A=xsimd::sse4_2
        ]
test.cpp(4): note: see reference to class template instantiation 'xsimd::batch<int8_t,xsimd::sse4_2>' being compiled
C:\tools\cmder\xsimd\include\xsimd\types\./xsimd_batch.hpp(25): error C2027: use of undefined type 'xsimd::types::simd_register<T,A>'
        with
        [
            T=int8_t,
            A=xsimd::sse4_2
        ]
C:\tools\cmder\xsimd\include\xsimd\types\./xsimd_batch.hpp(23): note: see declaration of 'xsimd::types::simd_register<T,A>'
        with
        [
            T=int8_t,
            A=xsimd::sse4_2
        ]
C:\tools\cmder\xsimd\include\xsimd\types\./xsimd_batch.hpp(29): error C2027: use of undefined type 'xsimd::types::simd_register<T,A>'
        with
        [
            T=int8_t,
            A=xsimd::sse4_2
        ]
C:\tools\cmder\xsimd\include\xsimd\types\./xsimd_batch.hpp(23): note: see declaration of 'xsimd::types::simd_register<T,A>'
        with
        [
            T=int8_t,
            A=xsimd::sse4_2
        ]
C:\tools\cmder\xsimd\include\xsimd\types\./xsimd_batch.hpp(29): error C2061: syntax error: identifier 'register_type'
C:\tools\cmder\xsimd\include\xsimd\types\./xsimd_batch.hpp(37): error C2061: syntax error: identifier 'register_type'
C:\tools\cmder\xsimd\include\xsimd\types\./xsimd_batch.hpp(37): error C2535: 'xsimd::batch<int8_t,xsimd::sse4_2>::batch(void)': member function already defined or declared
C:\tools\cmder\xsimd\include\xsimd\types\./xsimd_batch.hpp(33): note: see declaration of 'xsimd::batch<int8_t,xsimd::sse4_2>::batch'

[cyb70289]

This is a bit rude to always enable SSE4_2 for MSVC.
Per my understanding, 64bit MSVC supports at least SSE2, (/arch:SSE2 is actually an invalid option), and looks there's no way to detect SSE3/4 at compiler time.

[cyb70289]

Obviously this change causes windows-clang ci job failures.
Comments welcomed.

[cyb70289]

I'm puzzled: I activated SSE2 testing here: #563 but it doesn't show any error

Not familiar with appveyor environment. Just blind guess. From the SSE job log, looks it's still running AVX2 tests?
https://ci.appveyor.com/project/xtensor-stack/xsimd/builds/40755892/job/yn99tj388o4g2ku3#L736

[cyb70289]

Close this PR. Will be fixed by #563.