child_process: improve spawn performance on Linux

Speed up child_process.spawn by enabling the new V8 build flag which
makes fork/exec faster.

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

Author: kvakil

tools/v8_gypfiles/features.gypi

@@ -72,7 +72,21 @@
         'v8_enable_etw_stack_walking': 1,
       }, {
         'v8_enable_etw_stack_walking': 0,
-      }]
+      }],
+      ['OS=="linux"', {
+        # Sets -dV8_ENABLE_PRIVATE_MAPPING_FORK_OPTIMIZATION.
+        #
+        # This flag speeds up the performance of fork/execve on Linux systems for
+        # embedders which use it (like Node.js). It works by marking the pages that
+        # V8 allocates as MADV_DONTFORK. Without MADV_DONTFORK, the Linux kernel
+        # spends a long time manipulating page mappings on fork and exec which the
+        # child process doesn't generally need to access.
+        #
+        # See v8:7381 for more details.
+        'v8_enable_private_mapping_fork_optimization': 1,
+      }, {
+        'v8_enable_private_mapping_fork_optimization': 0,
+      }],
     ],
     'is_debug%': 0,
 
@@ -320,6 +334,9 @@
       ['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',],
       }],