Use memmove instead of memcpy for the large tensors on Linux.

Issue: #17246

~1.7x speedup for fetching a variable

Before:
  fetch_cpu_variable  : 5.5 GB/sec, min: 14.56, median: 15.05, mean: 15.14
  fetch_cpu_variable_add: 11.0 GB/sec, min: 7.29, median: 12.03, mean: 12.56
  fetch_cpu_variable_concat: 11.6 GB/sec, min: 6.92, median: 13.78, mean: 14.76

After:
  fetch_cpu_variable  : 9.2 GB/sec, min: 8.71, median: 8.79, mean: 8.80
  fetch_cpu_variable_add: 12.5 GB/sec, min: 6.41, median: 7.20, mean: 7.51
  fetch_cpu_variable_concat: 12.7 GB/sec, min: 6.32, median: 6.54
PiperOrigin-RevId: 199497691