wrong "catch exception" error message when finding trampoline msym.

When building the program with the shared GNAT runtime, the debugger
is unable to insert Ada exception catchpoints until that runtime
has been mapped to memory. In other words, we expect the user to start
the program first, before attempting to insert that catchpoint.

The detection mechanism that tries to provide some useful tips to
the user fails when the program itself contains a trampoline symbol
matching the symbol that the catchpoint is trying to use. This
results in the following error message:

    (gdb) catch exception
    Your Ada runtime appears to be missing some debugging information.
    Cannot insert Ada exception catchpoint in this configuration.

Instead, we expected the following error message:

    (gdb) catch exception
    Unable to insert catchpoint. Try to start the program first.

gdb/ChangeLog:

        * ada-lang.c (ada_has_this_exception_support): Ignore
        mst_solib_trampoline minimal symbols.
[ELinOS] Best effort to load system libraries in case of incomplete env

So far elinos.py was assuming that the whole ELinOS environment was
around to find the system libraries; if some environment variables
were missing, the script would just abort.

This was a bit extreme. It is possible to do better than that: to get
the core system libraries, one doesn't need to have a full environment
but just the path to the CDK. The path to kernel project is only
needed for the optional Xenomai libs.

gdb/ChangeLog:

	* system-gdbinit/elinos.py (get_elinos_environment): Return an
	incomplete dictionnary instead of None in case of missing
	environment variables.
	(elinos_init): in case of an incomplete environment, best
	effort to load system libraries instead of abort.