time: Avoid alignment gaps in __tzfile_read

By ordering the suballocations by decreasing alignment, alignment
gaps can be avoided.

Also use __glibc_unlikely for reading the transitions and type
indexes.  In the 8-byte case, two reads are now needed because the
transitions and type indexes are no longer adjacent.  The separate
call to __fread_unlocked does not matter from a performance point of
view because __tzfile_read is only invoked rarely.