parisc/unaligned: Use EFAULT fixup handler in unaligned handlers

Convert the inline assembly code to use the automatic EFAULT exception
handler. With that the fixup code can be dropped.

The other change is to allow double-word only when a 64-bit kernel is
used instead of depending on CONFIG_PA20.

Signed-off-by: Helge Deller <deller@gmx.de>