Factor some methods that were in DynamicLoaderPOSIXDYLD.

Move some code that was in DynamicLoaderPOSIXDLYD into the
base class DynamicLoader.  In the case of UpdateLoadedSections(),
the test to see whether a file is loadable (its address is zero)
is not generally applicable so that test is changed to a more
universally applicable check for the SHF_ALLOC flag on the section.

Also make it explicit that the reading of the module_id in
DynamicLoaderPOSIXDYLD::GetThreadLocalData() is using a hardcoded
size (of module_id) of 4, which might not be appropriate on
big-endian 64-bit systems, leaving a FIXME comment in place.