Fix read and write of float and double on big endian platforms

Change the pack/unpack format codes 'f' -> 'g' and 'd' -> 'e'
which ensure little endian format is used on the wire regardless
of machine endianness.

According to the PHP documentation, 'g' and 'e' were added in
PHP versions 7.0.15 and 7.1.1.

php/src/Google/Protobuf/Internal/GPBWire.php

@@ -241,7 +241,7 @@ class GPBWire
         if (!$input->readRaw(4, $data)) {
             return false;
         }
-        $value = unpack('f', $data)[1];
+        $value = unpack('g', $data)[1];
         return true;
     }
 
@@ -251,7 +251,7 @@ class GPBWire
         if (!$input->readRaw(8, $data)) {
             return false;
         }
-        $value = unpack('d', $data)[1];
+        $value = unpack('e', $data)[1];
         return true;
     }
 
@@ -360,13 +360,13 @@ class GPBWire
 
     public static function writeFloat(&$output, $value)
     {
-        $data = pack("f", $value);
+        $data = pack("g", $value);
         return $output->writeRaw($data, 4);
     }
 
     public static function writeDouble(&$output, $value)
     {
-        $data = pack("d", $value);
+        $data = pack("e", $value);
         return $output->writeRaw($data, 8);
     }