wrong predef flag usage

Working on a new boost release I found the lockfree queue stopped working proper and tracked it down: some x86_64 codes are included improperly in a couple of files, namely in boost/lockfree/detail/prefix.hpp and tagged_ptr_ptrcompression.hpp:

#ifdef BOOST_ARCH_X86_64

Should be

#if BOOST_ARCH_X86_64

Otherwise codes on all the other ARCHs will include the wrong piece. I tested the change on powerpc and it works fine so far.

Inspect boost/lockfree/detail/prefix.hpp and tagged_ptr_ptrcompression.hpp, starting with the BOOST_ARCH_X86_64 conditionals described in the issue. Update the conditional usage as specified, then verify the lockfree queue behavior on a non-x86_64 architecture or run the existing lockfree test suite.

Written by the indexing model from the issue text.