x86/ftrace: Get rid of function_hook

History lesson courtesy of Steve:

"When ftrace first was introduced to the kernel, it used gcc's
mcount profiling mechanism. The mcount mechanism would add a call to
"mcount" at the start of every function but after the stack frame was
set up. Later, in gcc 4.6, gcc introduced -mfentry, that would create a
call to "__fentry__" instead of "mcount", before the stack frame was
set up. In order to handle both cases, ftrace defined a macro
"function_hook" that would be either "mcount" or "__fentry__" depending
on which one was being used.

The Linux kernel no longer supports the "mcount" method, thus there's
no reason to keep the "function_hook" define around. Simply use
"__fentry__", as there is no ambiguity to the name anymore."

Drop it everywhere.

Signed-off-by: Borislav Petkov <bp@suse.de>
Acked-by: Jiri Slaby <jslaby@suse.cz>
Cc: "H. Peter Anvin" <hpa@zytor.com>
Cc: Ingo Molnar <mingo@redhat.com>
Cc: Jonathan Corbet <corbet@lwn.net>
Cc: Josh Poimboeuf <jpoimboe@redhat.com>
Cc: linux-doc@vger.kernel.org
Cc: linux-kernel@vger.kernel.org
Cc: Peter Zijlstra <peterz@infradead.org>
Cc: "Steven Rostedt (VMware)" <rostedt@goodmis.org>
Cc: Thomas Gleixner <tglx@linutronix.de>
Cc: x86@kernel.org
Link: http://lkml.kernel.org/r/20191018124800.0a7006bb@gandalf.local.home

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