This patch addresses upstream issues 5331
<https://svn.boost.org/trac/boost/attachment/ticket/5331> and 6692
<https://svn.boost.org/trac/boost/attachment/ticket/6692> by removing
the use of the deprecated swp{b} instructions on ARMv6 and ARMv7
architectures, replacing them with the newer {ld,st}rex.