ANDROID: add support for vendor hooks

Add support for vendor hooks. Adds include/trace/hooks
directory for trace definition headers where hooks
can be defined and vendor_hook.c for instantiating
and exporting them for vendor modules.

There are two variants of vendor hooks, both based
on tracepoints:

Normal: this uses the DECLARE_HOOK macro
to create a tracepoint function with the name trace_<name>
where <name> is the unique identifier for the trace.

Restricted: restricted hooks are needed for cases like
scheduler hooks where the attached function must be
called even if the cpu is offline or requires a
non-atomic context. Restricted vendor hooks cannot
be detached, so modules that attach to a restricted
hook can never unload. Also, only 1 attachment is
allowed (any other attempts to attach will fail with
-EBUSY).

For either case, modules attach to the hook by using
register_trace_<name>(func_ptr, NULL).

New hooks should be defined in headers in the
include/trace/hooks/ directory using the
DECLARE_HOOK() or DECLARE_RESTRICTED_HOOK()
macros.

New files added to include/trace/hooks should
be #include'd from drivers/android/vendor_hooks.c.
The EXPORT_TRACEPOINT_SYMBOL_GPL() should be
also added to drivers/android/vendor_hooks.c.

For example, if a new hook, 'android_vh_foo(int &ret)'
is added in do_exit() in exit.c, these changes are
needed:

1. create a new header file include/trace/hooks/foo.h
which contains:
	#include <trace/hooks/vendor_hooks.h>
	...
 	DECLARE_HOOK(android_vh_foo,
		     TP_PROTO(int *retp),
		     TP_ARGS(retp);

2. in exit.c, add
	#include <trace/hooks/foo.h>
	...
  	int ret = 0;
	...
  	android_vh_foo(&ret);
  	if (ret)
    		return ret;
	...

3. in drivers/android/vendor_hooks.c, add
	#include <trace/hooks/foo.h>
	...
	EXPORT_TRACEPOINT_SYMBOL_GPL(android_vh_foo);

The hook can then be attached by adding the registration code
to the module:

	#include <trace/hooks/sched.h>
	...
	static void my_foo(int *retp)
	{
		*retp = 0;
	}
	...
	rc = register_trace_android_vh_sched_exit(my_foo, NULL);

Bug: 156285741
Signed-off-by: Todd Kjos <tkjos@google.com>
Change-Id: I6a7d1c8919dae91c965e2a0450df50eac2d282db