Don't use catomic functions in mcount (BZ #16912)

mcount cannot use catomic functions since it is called by
__libc_start_main before TLS is set up.  This reverts the change made by
commit 8099361.
PowerPC: strncpy/stpncpy optimization for PPC64/POWER7

The optimization is achieved by following techniques:
  > data alignment [gain from aligned memory access on read/write]
  > POWER7 gains performance with loop unrolling/unwinding
    [gain by reduction of branch penalty].
  > zero padding done by calling optimized memset