sh4: fix toolchain creation

The Linux kernel does force compile with -m4-nofpu, which is only
available when building a multilib toolchain.
The interesting part here is, that buildroot use --disable-multilib for
gcc configure, but enables --with-multilib-list=m4,m4-nofpu in
the default configuration for Qemu targeting r2d emulation.
This results in a toolchain, which can be used for the kernel and
for userland without creating a multilib toolchain with different
kinds of libgcc version. In the multilib case there would be
subdirectories created (!m4 and m4-nofpu). As buildroot uses a
short version of toolchain creation, a multilib enabled gcc build
fails when creating libgcc.

So the best solution is to just keep multilib disabled, but always
add --with-multilib-list when sh4/sh4eb/sh4a/sh4aeb is choosen.

Tested with sh4/sh4a toolchain build and qemu defconfig with
gcc 4.8.x/4.9.x (with and without C++ enabled), uClibc and glibc.

Disable sh4a/sh4aeb for uClibc, as it does not implemented, yet.

Signed-off-by: Waldemar Brodkorb <wbx@openadk.org>
Signed-off-by: Thomas Petazzoni <thomas.petazzoni@free-electrons.com>
Tested-by: Thomas Petazzoni <thomas.petazzoni@free-electrons.com>
 (ARM and SH4 uClibc toolchain builds)