Fix x86 SIMD Extension Handling

[BMagnu]

This fixes a few things in our build handling related to SIMD instructions:

1. Fix detection of AVX2 capabilities
2. Include a check for OS-support on top of CPU-support for the extensions
3. Prevent MSVC from changing the extension set level in some circumstances. If the requested extension set is unavailable, just don't compile.
4. Actually consider the detected / set instruction set extensions on gcc / clang.

This means that the current behaviour is basically as follows:

1. On default settings, compile with march=native
2. This can be toggled off, in which case the compiler will only use the base x86_64 instructions + whatever was detected on the host system out of SSE/SSE2/AVX/AVX2
3. An instruction set can be forced, overriding any native CPU detection and capabilities.

Additionally, this PR will expose the availability of instruction sets to the code as defines, which I intend to make use of in the future.

Should probably be merged after #6375