enable V8 flag to MADV_DONTFORK V8's heap

Here are the results of running the existing benchmark. Note that this
optimization helps more for applications with larger heaps, so this is
somewhat of an underestimate of the real world performance benefits.

```console
$ ./node benchmark/compare.js --runs 15 \
        --new ./node \
        --old ~/node-v20/out/Release/node \
        --filter params child_process > cpr
$ node-benchmark-compare cpr
                                 confidence improvement  (***)
methodName='exec' n=1000                ***     60.84 % ±5.43%
methodName='execFile' n=1000            ***     53.72 % ±3.33%
methodName='execFileSync' n=1000        ***      9.10 % ±0.84%
methodName='execSync' n=1000            ***     10.44 % ±0.97%
methodName='spawn' n=1000               ***     53.10 % ±2.90%
methodName='spawnSync' n=1000           ***      8.64 % ±1.22%

  0.01 false positives, when considering a 0.1% risk acceptance (***)
```

Fixes: nodejs#25382
Fixes: nodejs#14917
Refs: nodejs/performance#93
Refs: nodejs/performance#89

tools/v8_gypfiles/features.gypi

@@ -111,6 +111,11 @@
     # Sets -dENABLE_HUGEPAGE
     'v8_enable_hugepage%': 0,
 
+    # Sets -dV8_ENABLE_PRIVATE_MAPPING_FORK_OPTIMIZATION. This speeds up
+    # fork/execve on Linux systems by marking V8's allocated pages as
+    # MADV_DONTFORK.
+    'v8_enable_private_mapping_fork_optimization%': 1,
+
     # Sets -dENABLE_VTUNE_JIT_INTERFACE.
     'v8_enable_vtunejit%': 0,
 
@@ -320,6 +325,9 @@
       ['v8_enable_hugepage==1', {
         'defines': ['ENABLE_HUGEPAGE',],
       }],
+      ['v8_enable_private_mapping_fork_optimization==1', {
+        'defines': ['V8_ENABLE_PRIVATE_MAPPING_FORK_OPTIMIZATION'],
+      }],
       ['v8_enable_vtunejit==1', {
         'defines': ['ENABLE_VTUNE_JIT_INTERFACE',],
       }],