[AArch64][SME] Disable NEON in streaming mode

In streaming mode most of the NEON instruction set is illegal, disable
NEON when compiling with `+streaming-sve`, unless NEON is explictly
requested.

Subsequent patches will add support for the small subset of NEON
instructions that are legal in streaming mode.

Reviewed By: paulwalker-arm, david-arm

Differential Revision: https://reviews.llvm.org/D107902