Allow configuration of the GC log file via an environment variable

The environment variable ES_GC_LOG_FILE contains the full path to the GC log file, all referenced
directories must already exist. If the variable is not set but GC logging is requested via ES_USE_GC_LOGGING
then the log file will be written to $ES_HOME/logs/gc.log

Closes #8471 

Note that the default name of the file was already proper for Windows, so the fix for #8471 effectively is "do what windows did".