For xonly, move sha512-ppc.pl's table from text to rodata

For xonly, move sha512-ppc.pl's table from text to rodata

OpenBSD/macppc will enforce xonly on PowerPC G5, then libcrypto's
sha256 would crash by SIGSEGV, because it can't read text.

Use ELF relocations "@ha" and "@l" to find the table in rodata.  This
might break the PowerPC asm on a not-ELF platform (like AIX or Mac OS)
if someone would try it there.

ok kettenis@ deraadt@