[OS/2] Use LibLoadExceptq instead of LoadExceptq.

NSPR is a separate DLL so this is to make sure that EXCEPTQ is only
installed if the main application has installed it and also to obey
the main app's EXCEPTQ options.
[OS/2] Generate INT 3 in MOZ_CRASH macro.

This is to better distinguish it from "regular" crashes in trap logs.
The synthetic access violation is still there for the case if the
INT 3 exception is somehow ignored by the exception handler.
[OS/2] Add EXCEPTQ support to chromium threads.