powerpc: Add new CPU feature: CPU_FTR_UNALIGNED_LD_STD

Add a new CPU feature bit, CPU_FTR_UNALIGNED_LD_STD, to be added
to the 64bit powerpc chips that can do unaligned load double and
store double without any performance hit.

This is added to Power6 and Cell and will be used in the next commit
to disable the code that gets the destination address aligned on
those CPUs where doing that doesn't improve performance.

Signed-off-by: Mark Nelson <markn@au1.ibm.com>
Signed-off-by: Paul Mackerras <paulus@samba.org>