log-tc-size is 24K by default. Page size is 64K on PPC64. But log-tc-
size
must be at least 3 x page size. This is enforced by TC_LOG_MMAP::open()
with a comment: to guarantee non-empty pool.

This all makes server not startable in default configuration on PPC64.

Autosize log-tc-size, so that it's min value= page size * 3, default
value= page size * 6, block size= page size.

Conflicts:
	mysql-test/suite/sys_vars/inc/sysvars_server.inc
	mysql-test/suite/sys_vars/r/sysvars_server_notembedded.result
	mysql-test/t/mysqld--help.test
	sql/mysqld.cc
	sql/sys_vars.cc