disable aarch64 test on travis CI

disable aarch64 test on travis CI

there's a bug on travis' ld
> collect2: error: ld terminated with signal 11 [Segmentation fault], core dumped

I cannot reproduce it.
Note that travis' ld version is 2.24
while the one in my Linux VM is 2.26.1.
ld is part of binutils, so it's pretty hard to change.

I would expect the bug to no longer be triggered after some random code change.
To be re-enabled later.