Make sure all functions in the hot path of eg_utf8_to_utf16_general are inlined.

This gives a 33% performance boost on a micro benchmark just triggering a call
to mono_string_new.
Make sure to unlock InternalThread::synch_cs when we fail to suspend the thread during shutdown.

This should fix both crashes seen during Environment.Exit by xbuild and possibly deadlocks.
Replace a useless g_assert with a g_error that actually gives us some information.
Add a bunch of dummy fields to Assembly under MOBILE to match the native layout.