Supporting data for golang/go#80899.
This is not a benchmark of CL 692295. It's a minimal, standalone
micro-benchmark isolating just the loop shape question — paired scalar
LDP/STP (copyGP, matching runtime.memmove's current arm64
implementation) vs. a single 4-register NEON VLD1.P/VST1.P per 64-byte
chunk (copyNEON) — with no alignment fixup, no software pipelining, and
no overlap handling. src/dst are fixed, page-resident arenas shared by