Harden gdb.base/coredump-filter.exp

Harden gdb.base/coredump-filter.exp

This testcase seems to assume the target is running Linux, so bare metal,
simulators and other debugging stubs running different OS' will have a
hard time executing some of the commands the testcase issues.

Even restricting the testcase to Linux systems (which the patch below does),
there are still problems with, say, QEMU not providing PID information when
"info inferior" is issued. As a consequence, the subsequent tests will either
fail or will not make much sense.

The attached patch checks if PID information is available. If not, it just
bails out and avoids running into a number of failures.

gdb/testsuite/ChangeLog:
2015-04-13  Luis Machado  <lgustavo@codesourcery.com>

	* gdb.base/coredump-filter.exp: Restrict test to Linux systems only.
	Handle the case of targets that do not provide PID information.