arm64: ftrace: Add dynamic ftrace support

This patch allows "dynamic ftrace" if CONFIG_DYNAMIC_FTRACE is enabled.
Here we can turn on and off tracing dynamically per-function base.

On arm64, this is done by patching single branch instruction to _mcount()
inserted by gcc -pg option. The branch is replaced to NOP initially at
kernel start up, and later on, NOP to branch to ftrace_caller() when
enabled or branch to NOP when disabled.
Please note that ftrace_caller() is a counterpart of _mcount() in case of
'static' ftrace.

More details on architecture specific requirements are described in
Documentation/trace/ftrace-design.txt.

Change-Id: I55b7a75c4e123dc4de9b904ff2689717d7df14ab
Acked-by: Will Deacon <will.deacon@arm.com>
Signed-off-by: AKASHI Takahiro <takahiro.akashi@linaro.org>
Signed-off-by: Will Deacon <will.deacon@arm.com>
Git-commit: bd7d38dbdf356e75eb3b1699158c9b8021fd6784
Git-repo: https://git.kernel.org/cgit/linux/kernel/git/torvalds/linux.git


[pratikp@codeaurora.org: fix compile error in
 arch/arm64/ftrace.c: ftrace_dyn_arch_init by changing the
 function argument from void to void *data]
Signed-off-by: Pratik Patel <pratikp@codeaurora.org>