immintrin.h is the de-factor standard header for clang and GCC to
include Intel intrinsics. Using this header avoids requiring the
compiler to use the `-maes` and `-msse4.1` compiler options on systems
that may not have AES or SSE instruction support.

clang: As seen in
https://github.com/llvm-mirror/clang/blob/master/lib/Headers/immintrin.h,
specific intrinsic header files are conditionally included depending on
whether the feature is available.

gcc: As seen in
https://github.com/gcc-mirror/gcc/blob/master/gcc/config/i386/immintrin.h,
gcc includes all intrinsic header files, but each individual file guards
against the feature not being available.

This came out of an investigation in
https://github.com/grpc/grpc/pull/27121.