m68knommu: fix user a5 register being overwritten

On no-MMU systems the application a5 register can be overwitten with the
address of the process data segment when processing application signals.
For flat format applications compiled with full absolute relocation this
effectively corrupts the a5 register on signal processing - and this very
quickly leads to process crash and often takes out the whole system with
a panic as well.

This has no effect on flat format applications compiled with the more
common PIC methods (such as -msep-data). These format applications reserve
a5 for the pointer to the data segment anyway - so it doesn't change it.

A long time ago the a5 register was used in the code packed into the user
stack to enable signal return processing. And so it had to be restored on
end of signal cleanup processing back to the original a5 user value. This
was historically done by saving away a5 in the sigcontext structure. At
some point (a long time back it seems) the a5 restore process was changed
and it was hard coded to put the user data segment address directly into a5.
Which is ok for the common PIC compiled application case, but breaks the
full relocation application code.

We no longer use this type of signal handling mechanism and so we don't
need to do anything special to save and restore a5 at all now. So remove the
code that hard codes a5 to the address of the user data segment.

Signed-off-by: Greg Ungerer <gerg@linux-m68k.org>