Retrieving data from compressed MyISAM table which is bigger than 4G on 32-bit box
with mmap() support results in server crash.

mmap() accepts length of bytes to be mapped in second param, which is 32-bit
size_t. But we pass data_file_length, which is 64-bit my_off_t. As a result only
first data_file_length % 4G were mapped.

This fix adds additional condition for mmap() usage, that is use mmap() for
compressed table which size is no more than 4G on 32-bit platform.