Enable uint64 compareRanks on arm64

Extend the uint64 rank comparison trick (comparing [4]uint16 as a
single uint64) to arm64 builds. ARM64 is little-endian like x86, so
the same unsafe.Pointer cast produces correct lexicographic ordering.

This replaces a 4-iteration loop with a single uint64 comparison,
speeding up the sort phase.

Chromium file list, single-threaded:
  linux:  126ms -> 126ms (sort not dominant)
  src:    462ms -> 438ms (-5%, sort-heavy)

src/result_others.go

@@ -1,4 +1,4 @@
-//go:build !386 && !amd64
+//go:build !386 && !amd64 && !arm64
 
 package fzf
 

src/result_x86.go

@@ -1,4 +1,4 @@
-//go:build 386 || amd64
+//go:build 386 || amd64 || arm64
 
 package fzf