BFD error message suppression test case

BFD error message suppression test case

This commit adds a GDB test case which tests GDB's BFD error handler
hook for suppressing output of all but the first identical messages.

See the comment at the beginning of bfd-errors.exp for details about
this new test.

I've tested this test for both 32- and 64-bit ELF files and also
on both little endian and big endian machines.  It also works for
both native and remote targets.  The only major restriction is that
it only works for ELF targets.