[S390] fix s390x_newuname

The uname system call for 64 bit compares current->personality without
masking the upper 16 bits. If e.g. READ_IMPLIES_EXEC is set the result
of a uname system call will always be s390x even if the process uses
the s390 personality.

Signed-off-by: Martin Schwidefsky <schwidefsky@de.ibm.com>