[CMake] Specify 32-bit compilation to Clang

[Nils Gladitz]

I am not sure what else it could be.
Here are some queries I did on my system where it seems to be working; 
perhaps it will help you backtrack the issue:

Ubuntu 14.04.1 LTS (64-bit)

# gcc --version :
gcc (Ubuntu 4.8.2-19ubuntu1) 4.8.2

# clang --version :
Ubuntu clang version 3.4-1ubuntu3 (tags/RELEASE_34/final) (based on LLVM 
3.4)

# clang++ -m32 -Wl,--verbose test.cpp|grep libgcc
attempt to open /usr/bin/../lib/gcc/x86_64-linux-gnu/4.8/32/libgcc_s.so 
succeeded
-lgcc_s (/usr/bin/../lib/gcc/x86_64-linux-gnu/4.8/32/libgcc_s.so)

# file a.out
a.out: ELF 32-bit LSB  executable, Intel 80386, version 1 (SYSV), 
dynamically linked (uses shared libs), for GNU/Linux 2.6.24, 
BuildID[sha1]=e15359bda06cce657369b3ae9d526c926deea6f2, not stripped

# apt-file search /usr/lib/gcc/x86_64-linux-gnu/4.8/32/libgcc_s.so
lib32gcc-4.8-dev: /usr/lib/gcc/x86_64-linux-gnu/4.8/32/libgcc_s.so

g++-multilib has an indirect dependency on lib32gcc-4.8-dev.

Nils