Add a check for required extensions in benchmark.php (php#14811)

This should prevent people from being disappointed by
an unexpected error message when running the benchmark,
namely:

```
jvoisin@chernabog 21:54 (master) ~/dev/php-src php ./benchmark/benchmark.php false sapi/cgi/php-cgi
> 'valgrind' '--tool=callgrind' '--dump-instr=yes' '--callgrind-out-file=/home/jvoisin/dev/php-src/benchmark/profiles/callgrind.out.bench' '--' 'sapi/cgi/php-cgi' '-T1' '-d max_execution_time=0' '-d opcache.enable=1' '-d opcache.jit=disable' '-d opcache.jit_buffer_size=128M' '-d opcache.validate_timestamps=0' '/home/jvoisin/dev/php-src/Zend/bench.php'
> 'valgrind' '--tool=callgrind' '--dump-instr=yes' '--callgrind-out-file=/home/jvoisin/dev/php-src/benchmark/profiles/callgrind.out.bench.jit' '--' 'sapi/cgi/php-cgi' '-T1' '-d max_execution_time=0' '-d opcache.enable=1' '-d opcache.jit=tracing' '-d opcache.jit_buffer_size=128M' '-d opcache.validate_timestamps=0' '/home/jvoisin/dev/php-src/Zend/bench.php'
> '/usr/bin/php' '/home/jvoisin/dev/php-src/benchmark/repos/symfony-demo-2.2.3/bin/console' 'cache:clear'
> '/usr/bin/php' '/home/jvoisin/dev/php-src/benchmark/repos/symfony-demo-2.2.3/bin/console' 'cache:warmup'
> 'valgrind' '--tool=callgrind' '--dump-instr=yes' '--callgrind-out-file=/home/jvoisin/dev/php-src/benchmark/profiles/callgrind.out.symfony-demo' '--' 'sapi/cgi/php-cgi' '-T50,50' '-d max_execution_time=0' '-d opcache.enable=1' '-d opcache.jit=disable' '-d opcache.jit_buffer_size=128M' '-d opcache.validate_timestamps=0' '/home/jvoisin/dev/php-src/benchmark/repos/symfony-demo-2.2.3/public/index.php'
PHP Fatal error:  Uncaught Error: Call to undefined function gmp_strval() in /home/jvoisin/dev/php-src/benchmark/benchmark.php:128
Stack trace:
  thrown in /home/jvoisin/dev/php-src/benchmark/benchmark.php on line 128
[255]
jvoisin@chernabog 12:05 (master) ~/dev/php-src
```

benchmark/benchmark.php

@@ -2,6 +2,12 @@
 
 require_once __DIR__ . '/shared.php';
 
+foreach (array("mbstring", "sockets", "mysqli", "openssl", "gmp") as $ext) {
+    if (!extension_loaded($ext)) {
+        throw new LogicException("Extension $ext is required.");
+    }
+}
+
 $storeResult = ($argv[1] ?? 'false') === 'true';
 $phpCgi = $argv[2] ?? dirname(PHP_BINARY) . '/php-cgi';
 if (!file_exists($phpCgi)) {