u-boot currently fails to build from source on sh4 since the
linker name prefix defined in arch/sh/config.mk is outdated.
On current Debian installations, the binaries of the GNU
toolchain are named using the triplett scheme
$arch-$kernel-$toolchain. Thus, on sh4, the proper name
is "sh4-linux-gnu-ld" instead of "sh4-linux-ld". This
patch updates build configuration on sh4 to reflect that.