toolchain: enable musl for sh

The SuperH architecture is supported by the musl libc since some time
now, so let's enable it.
Tested via qemu_sh4_r2d_defconfig.

Signed-off-by: Lionel Orry <lionel.orry@gmail.com>
Signed-off-by: Thomas Petazzoni <thomas.petazzoni@free-electrons.com>