[boost] [predef] BOOST_ARCH_BITS_* feature request

Hi, It looks like for low-level programming following macro could be really useful: BOOST_ARCH_BITS_64 BOOST_ARCH_BITS_32 BOOST_ARCH_BITS_16 ? Predef also missing BOOST_ARCH_ARM_64, BOOST_ARCH_PPC_64. Boost.DLL really needs those macro for detecting bit's dependent elf/macho/pe structures layout. Is it possible to add them? -- Best regards, Antony Polukhin