powerpc/32: Don't clobber personality flags on exec

Now that ppc32 implements address randomization it also wants to inherit
personality flags like ADDR_NO_RANDOMIZE across exec, for things like
`setarch ppc -R' to work.  But the ppc32 version of SET_PERSONALITY
forcefully sets PER_LINUX, clearing all personality flags.  So be
careful about preserving the flags.

Signed-off-by: Andreas Schwab <schwab@linux-m68k.org>
Signed-off-by: Paul Mackerras <paulus@samba.org>