Support PHP8

[Joungkyun]

PHP 8 support

• Fixed a problem that failed due to TSRM CONSTANT removed from PHP 8
• Modify the Miss Casting Warning of the second argument of Zend_Hash_INDEX_FIND
• All pass result of make test

  =====================================================================
  PHP         : /usr/bin/php80
  PHP_SAPI    : cli
  PHP_VERSION : 8.0.3
  ZEND_VERSION: 4.0.3
  PHP_OS      : Linux - Linux AnNyung build
  INI actual  : /root/work/github/libevent/tmp-php.ini
  More .INIs  :
  CWD         : /root/work/github/libevent
  Extra dirs  :
  VALGRIND    : Not used
  =====================================================================
  TIME START 2021-03-13 19:33:32
  =====================================================================
  PASS pecl/libevent - general [tests/libevent001.phpt]
  PASS pecl/libevent - bug https://github.com/expressif/pecl-event-libevent/issues/2 [tests/libevent002.phpt]
  PASS pecl/libevent - bug https://github.com/expressif/pecl-event-libevent/issues/3 [tests/libevent003.phpt]
  PASS pecl/libevent - bug https://github.com/expressif/pecl-event-libevent/issues/9 [tests/libevent004.phpt]
  PASS pecl/libevent - bug https://github.com/expressif/pecl-event-libevent/issues/8 [tests/libevent005.phpt]
  PASS pecl/libevent - bug https://github.com/expressif/pecl-event-libevent/issues/8 [tests/libevent006.phpt]
  PASS pecl/libevent - bug https://github.com/expressif/pecl-event-libevent/issues/14 [tests/libevent007.phpt]
  PASS pecl/libevent - bug https://github.com/expressif/pecl-event-libevent/issues/19 [tests/libevent008.phpt]
  =====================================================================
  TIME END 2021-03-13 19:33:39
  
  =====================================================================
  TEST RESULT SUMMARY
  ---------------------------------------------------------------------
  Exts skipped    :    0
  Exts tested     :   34
  ---------------------------------------------------------------------
  
  Number of tests :    8                 8
  Tests skipped   :    0 (  0.0%) --------
  Tests warned    :    0 (  0.0%) (  0.0%)
  Tests failed    :    0 (  0.0%) (  0.0%)
  Tests passed    :    8 (100.0%) (100.0%)
  ---------------------------------------------------------------------
  Time taken      :    7 seconds
  =====================================================================