parisc: Optimze cache flush algorithms

The attached patch implements three optimizations:

1) Loops in flush_user_dcache_range_asm, flush_kernel_dcache_range_asm,
purge_kernel_dcache_range_asm, flush_user_icache_range_asm, and
flush_kernel_icache_range_asm are unrolled to reduce branch overhead.

2) The static branch prediction for cmpb instructions in pacache.S have
been reviewed and the operand order adjusted where necessary.

3) For flush routines in cache.c, we purge rather flush when we have no
context.  The pdc instruction at level 0 is not required to write back
dirty lines to memory. This provides a performance improvement over the
fdc instruction if the feature is implemented.

Version 2 adds alternative patching.

The patch provides an average improvement of about 2%.

Signed-off-by: John David Anglin <dave.anglin@bell.net>
Signed-off-by: Helge Deller <deller@gmx.de>