Support for AVX MSVC builds

[mekhontsev]

To have <immintrin.h>, we need to check the SSE2 and AVX macros.

[saghul]

What problem did you run into?

[mekhontsev]

When I tried to compile xsum.c with AVX2 enabled (latest MSVC), I got the missing definitions from immintrin.h

[saghul]

IS that something we could add to the CI?

[mekhontsev]

Since QuickJS already uses SSE/AVX, it would be better to have at least one CMake target to check this code path...

[saghul]

I don't have much experience in that myself 😅 What do you have in mind?

[mekhontsev]

At the first stage we can just add the simplest check (something like):

if(MSVC)
    add_library(xsum xsum.c)
    target_compile_options(xsum PRIVATE /arch:AVX2)
endif()