Altivec support requires >= P7

Originally reported downstream in Gentoo at https://bugs.gentoo.org/920234.

We pass --enable-power-altivec if the user specifies they have altivec, but nettle's "altivec" asm requires P7:

    *powerpc64*)
      if test "$ABI" = 64 ; then
	GMP_ASM_POWERPC_R_REGISTERS
	asm_path="powerpc64"
	if test "x$enable_fat" = xyes ; then
	  asm_path="powerpc64/fat $asm_path"
	  OPT_NETTLE_SOURCES="fat-ppc.c $OPT_NETTLE_SOURCES"
	  FAT_TEST_LIST="none crypto_ext altivec power9"
	else
	  if test "$enable_power9" = yes ; then
	    asm_path="powerpc64/p9 $asm_path"
	  fi
    if test "$enable_power_crypto_ext" = yes ; then
            asm_path="powerpc64/p8 $asm_path"
	  fi
	  if test "$enable_power_altivec" = yes ; then
	    asm_path="powerpc64/p7 $asm_path"
	  fi
	fi
      fi
      ;;

This leads to illegal instruction errors on a PowerMac G5.

Could the check be made to check if __VSX__ is defined (=> at least P7) and otherwise fallback to no-asm? Thanks.