time64.h: Declare tm_zone as const under Android, but not if using C++

https://android.googlesource.com/platform/bionic.git/+/master/libc/include/time.h
time64.h: On QNX, use const for tm_zone

http://www.qnx.com/developers/docs/6.3.0SP3/neutrino/lib_ref/t/tm.html