[CMake] Pass -march=i686 when targeting i686 (PR24222)

Clang will not define __i686__, even when the target triple is i686,
without -march=i686.

With this patch, the compiler-rt build will successfully detect that
Clang can target i686.

The open_memstream.cc test is a little funny. Before my patch, it
was invoked with "-m32 -m64". To make it work after my -march
change, I had to add '-march=x86-64'.

Differential Revision: http://reviews.llvm.org/D11618

llvm-svn: 243604