Avoid crash with cross-linux core file

Avoid crash with cross-linux core file

An internal test case creates a core file using gcore, then restarts
gdb with that core.  When run with a cross-linux gdb (in this case,
x86-64 host with ppc64-linux target), the test fails:

    | (gdb) core core
    | [New LWP 18437]
    | warning: `/lib64/libc.so.6': Shared library architecture unknown is not compatible with target architecture powerpc:common64.
    | warning: Could not load shared library symbols for /lib64/ld64.so.1.
    | Do you need "set solib-search-path" or "set sysroot"?
    | ../../src/gdb/gdbarch.c:3388: internal-error: int gdbarch_elf_make_msymbol_special_p(gdbarch*): Assertion `gdbarch != NULL' failed.
    | A problem internal to GDB has been detected,
    | further debugging may prove unreliable.
    | Quit this debugging session? (y or n) y

What's happening here is that the core file lists some shared
libraries.  These aren't available via the solib search path, and so
gdb finds the local (x86-64) libraries.  This is not ideal, but on the
other hand, it is what was asked for -- while the test does set
solib-search-path, it does not set the sysroot.

But, because gdb isn't configured to handle these libraries, it
crashes.

It seems to me that it's better to avoid the crash by having
solib_bfd_open fail in the case where a library is incompatible.  That
is what this patch does.  Now it looks like:

    | [New LWP 15488]
    | Error while mapping shared library sections:
    | `/lib64/libc.so.6': Shared library architecture unknown is not compatible with target architecture powerpc:common64.

... and does not crash gdb.

I don't have a good setup for testing this using dejagnu, so I don't
know whether an existing gdb test covers this scenario.