qemu: fix build on sh4a and sh4aeb

Qemu doesn't understand sh4a and sh4aeb as separate architectures, it
only knows about sh4 and sh4eb. However, it does support sh4a, since
it emulates the SH7785 CPU which is a SH4A.

Therefore, with this commit, sh4a/sh4aeb are translated as sh4/sh4eb
for Qemu.

Fixes:

  http://autobuild.buildroot.org/results/8f9/8f99a776f35d4ab0841d9fd2db509850ef603bd4/


  and many previous similar build failures

Signed-off-by: Thomas Petazzoni <thomas.petazzoni@free-electrons.com>
Signed-off-by: Peter Korsgaard <peter@korsgaard.com>