libobs,cmake: Replace vendored SIMD Everywhere by prefix/system install

SIMD Everywhere finder needs to be installed alongside libobs CMake
package since its headers depends on it.

C++ cmath header is included on Windows on ARM to ensure that all math
functions are correctly defined when included in C++ code.
https://github.com/simd-everywhere/simde/issues/1304

macOS intrinsics is included to make sure that no redefinition error
happens.