[libcxx] implement <simd> ABI for Clang/GCC vector extension, constructors, copy_from and copy_to.
This patch adds a new macro _LIBCPP_HAS_NO_VECTOR_EXTENSION for detecting
whether a vector extension (\_\_attribute\_\_((vector_size(num_bytes)))) is
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.
Reviewers: mclow.lists, EricWF
Subscribers: cfe-commits, MaskRay, lichray, sanjoy
Differential Revision: https://reviews.llvm.org/D41376