Add CPU arch conditions to test 05_32_bits
This should solve FTBFS on riscv64 as well as other platforms that
does not support -m32 or -m64 options.
.
Tested on Debian riscv64 on QEMU and worked fine.
.
I have also submitted a [PR](https://github.com/haampie/libtree/pull/82)
to upstream, but they haven't merged it yet.
.
I've only included part of platforms here, including x86_64 and
i386, since I don't know the exact output of uname -m on those
platforms. Feel free to append more platforms to the list.
