powerpc: Fix powerpc64 build failure with binutils 2.22

powerpc: Fix powerpc64 build failure with binutils 2.22

GLIBC memset optimization for POWER8 uses the '.machine power8'
directive, which is only supported officially on binutils 2.24+.  This
causes a build failure on older binutils.

Since the requirement of .machine power8 is to correctly assembly the
'mtvsrd' instruction and it is already handled by the MTVSRD_V1_R4
macro, there is no really needed of using it.

The patch replaces the power8 with power7 for .machine directive.

It fixes BZ#17869.