This patch added a new macro _LIBCPP_HAS_VECTOR_EXTENSION for detecting
whether a vector extension (__attribute__((vector_size(num_bytes)))) is
This patch also backports std::integer_sequence to C++11.
On the top of that, this patch implements the following API:
* all constructors
It also defines simd_abi::native to use vector extension, if available.
In GCC and Clang, certain values with vector extension are passed by registers,
instead of memory.
Based on D41148.