liblzma: memcmplen.h: Use 8-byte method on 64-bit unaligned archs

liblzma: memcmplen.h: Use 8-byte method on 64-bit unaligned archs

Previously it was enabled only on x86-64 and ARM64 when also support
for unaligned access was detected or manually enabled at built time.

In the default build configuration, the 8-byte method is now enabled
also on 64-bit RISC-V and 64-bit PowerPC (both endiannesses). It was
reported that on big endian POWER9, encoding time may reduce 12-13 %.

This change only affects builds with GCC and Clang because the code
uses __builtin_ctzll or __builtin_clzll.

Thanks to Marcus Comstedt for testing on POWER9.