In my understanding when you use intrinsics and build for a processor without support for the intrinsics then GCC for example will replace it with equivalent code.
That is the case with GCCs __builtin functions. With a few exceptions, intrinsics are basically macros for inline asm that the compiler can reason about.
If on x86-64 you use a _mm256* intrinsic and compile without AVX support you just get a compile error, not a pair of equivalent SSE instructions.
Second, if the intrinsics aren't built-in then there may be faster alternatives than using the GCC emulated version.