liblzma: Add ifunc implementation to crc64_fast.c.

liblzma: Add ifunc implementation to crc64_fast.c.

The ifunc method avoids indirection via the function pointer
crc64_func. This works on GNU/Linux and probably on FreeBSD too.
The previous __attribute((__constructor__)) method is kept for
compatibility with ELF platforms which do support ifunc.

The ifunc method has some limitations, for example, building
liblzma with -fsanitize=address will result in segfaults.
The configure option --disable-ifunc must be used for such builds.

Thanks to Hans Jansen for the original patch.
Closes: https://github.com/tukaani-project/xz/pull/53