With MICROPY_FLOAT_IMPL_FLOAT the results of utime.time(), gmtime() and
localtime() change only every 129 seconds. As one consequence
tests/extmod/vfs_lfs_mtime.py will fail on a unix port with LFS support.
With this patch these functions only return floats if
MICROPY_FLOAT_IMPL_DOUBLE is used. Otherwise they return integers.