[SystemZ] Emit a .gnu_attribute for an externally visible vector abi.

On SystemZ, the vector ABI changes depending on the presence of hardware
vector support. Therefore, each binary compiled with a visible vector ABI
(e.g. one that calls an external function with a vector argument) should be
marked with a .gnu_attribute describing this.

Reviewed By: uweigand

Differential Revision: https://reviews.llvm.org/D105067