Convert PLCrashAsyncObjCSection.c to C++; we'll use templates to handle the 32-bit/64-bit split.

This also fixes an issue caught when building C++; the
deprecated plcrash_async_read_addr() returns a kern_return_t,
but the result was interpreted as a plcrash_err_t (this is why
plcrash_async_read_addr() is now deprecated).

Issue: PLCR-495