I had the same issue, I wasn't able to find any official information anywhere so I ended up just ssh-ing into the ec2 instance that elastic beanstalk creates and running the right commands to determine the default installed php extensions.
I followed this answer to ssh into the ec2 instance: https://stackoverflow.com/a/4921866/8137812
And then I ran php -r "print_r(get_loaded_extensions());"
on the ec2 instance.
This gave me the list of all the default extensions that are installed, specifically for the "PHP 7.1 running on 64bit Amazon Linux/2.6.1" platform that AWS provides.
Array
(
[0] => Core
[1] => date
[2] => libxml
[3] => openssl
[4] => pcre
[5] => zlib
[6] => filter
[7] => hash
[8] => pcntl
[9] => readline
[10] => Reflection
[11] => SPL
[12] => session
[13] => standard
[14] => bcmath
[15] => bz2
[16] => calendar
[17] => ctype
[18] => curl
[19] => dom
[20] => mbstring
[21] => fileinfo
[22] => ftp
[23] => gd
[24] => gettext
[25] => iconv
[26] => intl
[27] => json
[28] => exif
[29] => mcrypt
[30] => mysqlnd
[31] => odbc
[32] => PDO
[33] => pgsql
[34] => Phar
[35] => posix
[36] => shmop
[37] => SimpleXML
[38] => soap
[39] => sockets
[40] => sqlite3
[41] => sysvmsg
[42] => sysvsem
[43] => sysvshm
[44] => tokenizer
[45] => xml
[46] => xmlwriter
[47] => xsl
[48] => zip
[49] => mysqli
[50] => pdo_mysql
[51] => PDO_ODBC
[52] => pdo_pgsql
[53] => pdo_sqlite
[54] => wddx
[55] => xmlreader
[56] => xmlrpc
[57] => apcu
[58] => igbinary
[59] => imagick
[60] => memcache
[61] => OAuth
[62] => ssh2
[63] => memcached
[64] => Zend OPcache
)