libexec/kgdb: Add new modules and install them together with debug info

This change simplifies integration of gdb python scripts with our kernel
debugging infrastructure. Rather than putting debugg

libexec/kgdb: Add new modules and install them together with debug info

This change simplifies integration of gdb python scripts with our kernel
debugging infrastructure. Rather than putting debugging scripts in
/usr/libexec/kgdb, move them to <path-to-kernel-debug-symbols>/gdb, and
add a kernel-gdb.py which automatically loads modules from that
directory. kernel-gdb.py will be automatically executed by kgdb when
loading kernel debug symbols (assuming a default configuration), so one
no longer needs to do anything to use these modules.

The change also adds a couple of new modules, vnet.py and pcpu.py, for
conveniently accessing VNET symbols and PCPU/DPCPU fields, respectively.
Note that these require a change to the kernel linker when accessing
symbols from a loadable kernel module.

sys/tools/gdb/README.txt describes the scheme in more detail and
provides some rudiementary documentation for the commands and functions
added by these modules. It should be updated when adding new features.

sys/tools/gdb/selftest.py can be used to do some primitive testing of
the modules. All it does is execute a number of gdb commands making use
of commands and functions added by these modules. The developer is
expected to verify that the commands complete without errors and that
the output looks sane.

Discussed with: kp, avg, jhb, glebius
MFC after: 2 weeks
Differential Revision: https://reviews.freebsd.org/D50825

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