There are several places in which we access negative offsets from
the thread-pointer, but thumb2 only supports positive offsets in
memory references.
Avoid duplicating the rather large macros in which these references
are embedded by abstracting out the operation.
This feature is specifically for the C++ compiler to offload calling
thread_local object destructors on thread program exit, to glibc.
This is to overcome the possible complication of destructors of
thread_local objects getting called after the DSO in which they're
defined is unloaded by the dynamic linker. The DSO is marked as
'unloadable' if it has a constructed thread_local object and marked as
'unloadable' again when all the constructed thread_local objects
defined in it are destroyed.
