powerpc: Fix msr check in compat_sys_swapcontext

The new context may not be 16-byte aligned, so the real address of the
mcontext structure should be read from the uc_regs pointer instead of
directly using the (unaligned) uc_mcontext field.

Signed-off-by: Andreas Schwab <schwab@suse.de>
Signed-off-by: Paul Mackerras <paulus@samba.org>