x86: Enable 32-bit build using x86_64 multilib toolchain

Until now building the x86 arch boards required 32-bit toolchain. As
many x86_64 toolchains come with 32-bit support (multilib) that's a
good idea to enable build with such toolchains.

The change required was to specify the usage of 32-bit explicitly to
the compiler and the linker (-m32 and -m elf_i386 flags) and locate
the right libgcc path.

Signed-off-by: Vasili Galka <vvv444@gmail.com>
Acked-by: Simon Glass <sjg@chromium.org>