Add 32 bit testing to continous integration

[pauldreik]

Rdfind 1.4.0 failed to build on some of the Debian's build machines. It worked on amd64, arm64, mips64, ppc64el, s390x, alpha, ia64, ppc64, riscv64 and sparc64. It failed on armel, armhf, i386,mips,hppa, hurd-i386,m68k, powerpc, powerpcspe, sh4 and x32. The common denominator is that all those are (I believe) 32 bit platforms.
This demonstrates that building on a single architecture (amd64) is not enough.
Here are some ideas how to fix this on appveyour and/or travis

• build on 32 bit x86 (how? through debian multiarch?)
• qemu ?
• docker?
• x32?

Is there any freely available continuous integration that uses arm? Thankful for input and suggestions.

[pauldreik]

I solved this with gcc multilib for compiling with -m32 on amd64.
It is now implemented on master, runs on appveyor.