linux: fix compiling under a 32-bit VM/64-bit arch

Fix build failure:

https://github.com/msantos/procket/issues/13

rebar designates 32-bit VM on x864_64 as
"R16B03-1-i686-pc-linux-gnu-32".

My guess is that a half-word emulator will be reported as
"x86_64.*-32", so try to accomodate this case as well.

Thanks @NAR!