Vector Function ABI specifications for AArch64 update

[Szabolcs Nagy]

Arm released an update (2019Q1.1) of the Vector Function ABI specifications for
AArch64 that uses the `declare variant` directive from OpenMP 5.0 to support user
defined vector functions. The mechanism is introduced in chapter 4, and it is in
beta release status to allow feedback from the open source community.
The mechanism also allows declaring SVE and AdvSIMD vector functions independently
which is not possible with the current OpenMP and attribute(simd) support in gcc.
Feedback needs to be provided at arm.eabi (at) arm.com by end of June 16th (AOE).

https://developer.arm.com/tools-and-software/server-and-hpc/arm-architecture-tools/arm-compiler-for-hpc/vector-function-abi

Thanks.