ARM64: add extra NEG pattern.
ARM64: don't emit .subsections_via_symbols on ELF.

Part of PR19455.
ARM64: disable generation of .loh directives outside MachO.

Part of PR19455.
Allocator: Remove ReferenceAdder hack.

This was a workaround for compilers that had issues with reference
collapsing.
AArch64/ARM64: add more NEON tests.

Mostly no testing this time, since they were just wrangling
target-specific intrinsics.