Fix PR gdb/20505 - Make vDSO detection work with core files

Fix PR gdb/20505 - Make vDSO detection work with core files

Loading a core dump that was either generated on a system running
pristine glibc master, or on a Fedora/RHEL system with LD_DEBUG=unused
set in the environment, solib-svr4.c:svr4_current_sos fails to filter
out the vDSO, resulting in:

  (gdb) core-file corefile.core^M
  [New LWP 2362]^M
  warning: Could not load shared library symbols for linux-vdso.so.1.^M
  Do you need "set solib-search-path" or "set sysroot"?^M
  Core was generated by `build-gdb/gdb/testsuite/outputs/gdb.base/corefile/'.^M
  ...

The problem is that gdbarch_vsyscall_range does not support core
inferiors at all.

When live debugging, we're finding the vDSO's start address with
auxv/AT_SYSINFO_EHDR, and then we find the vDSO's size by look for the
corresponding mapping, by parsing /proc/PID/maps.  When debugging a
core dump, we can also determine the starting address from
auxv/AT_SYSINFO_EHDR.  However, we obviously can't read the core
mappings out of the host's /proc.  But we can instead look for a
corresponding load segment in the core's bfd.

gdb/ChangeLog:
2016-08-22  Pedro Alves  <palves@redhat.com>

	PR gdb/20505
	* linux-tdep.c (linux_vsyscall_range_raw): For core inferiors,
	find the vDSO's start address with AT_SYSINFO_EHDR too, and
	determine the vDSO's size by finding the PT_LOAD segment that
	matches AT_SYSINFO_EHDR.

gdb/testsuite/ChangeLog:
2016-08-22  Pedro Alves  <palves@redhat.com>

	PR gdb/20505
	* gdb.base/vdso-warning.exp: Test core dumps too.  Use
	with_test_prefix.  Factor out bits to ...
	(test_no_vdso): ... this new procedure.