parisc: Add ALTERNATIVE_CODE() and ALT_COND_RUN_ON_QEMU

The macro ALTERNATIVE_CODE() allows assembly code to patch in a series
of new assembler statements given at a specific start address.
The ALT_COND_RUN_ON_QEMU condition is true if the kernel is started in a
qemu emulation.

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