gdb: skip objfiles with no BFD in DWARF unwinder

gdb: skip objfiles with no BFD in DWARF unwinder

While playing with JIT reader I experienced GDB to crash on null-pointer
dereference when stepping through non-jitted code.

The problem was that dwarf2_frame_find_fde () assumed that all objfiles
have BFD but that's not always true. To address this problem, this
commit skips such objfiles.

To test the fix we put breakpoint in jit_function_add (). The JIT reader
does not know how unwind this function so unwinding eventually falls
back to DWARF unwinder which in turn iterates over objfiles. Since the
the code is jitted, it is guaranteed it would eventually process JIT
objfile.

Approved-By: Simon Marchi <simon.marchi@efficios.com>