On Linux, allow shared-memory network weights to be backed by huge pages
This does require the user to have set `/sys/kernel/mm/transparent_hugepage/shmem_enabled` to something other than `never` (unfortunately the default, at least on Ubuntu).
```
Result of 50 runs
==================
base (...kfish.master) = 1814244 +/- 4396
test (./stockfish ) = 1823756 +/- 3448
diff = +9512 +/- 5496
speedup = +0.0052
P(speedup > 0) = 0.9996
```
```
Performance counter stats for './stockfish bench':
237,561 dTLB-misses
5,783,003,231 cycles
Performance counter stats for './bins/stockfish.master bench':
2,219,832 dTLB-misses
5,932,779,255 cycles
```
closes https://github.com/official-stockfish/Stockfish/pull/6805
No functional change