Dynamically linked code from shared libraries which may reference mode
checking wrappers cannot depend on symbols which are only visible from
the static link phase, such as those defined by the libmodechk.a
library. Otherwise, unresolved references to symbols from libmodechk.a
may exist in shared objects such as libcopperplate.so.

Unfortunately, we can't interpose on the cxa* API from a dynamic
library, since the latter would create references to "real"
__cxa_acquire/release/abort() routines, which are not available from
plain C builds since only libstdc++ defines them.

Providing placeholders for the cxa* API calls in a dynamic object is
not an option either, since that would create a dependency on the link
order between libmodechk and libstdc++ for pulling the actual symbols
instead of the dummy ones, which is not acceptable. Besides, relying
on LD_DYNAMIC_WEAK is fragile, unpractical, and won't work in secured
execution mode.

To work around this chicken-and-egg issue, make libmodechk a shared
object, and drop mode checking for the cxa* API for now. We'll revisit
the issue of enabling back the cxa* API later.