[aarch64] Enable `aarch64_vector_pcs` attribute.

This commit enables building `libsleef` and `libsleefgnuabi` with the
`aarch64_vector_pcs` attribute defined in the _Vector Function ABI
specification for AArch64_ [1].

The build must be configured with `-DFORCE_AAVPCS=On`. By default this
configure variable is set to `Off`.

[1] https://developer.arm.com/products/software-development-tools/hpc/arm-compiler-for-hpc/vector-function-abi