arm64: ftrace: use function_nocfi for ftrace_call

With CONFIG_CFI_CLANG, the compiler replaces function pointers with
jump table addresses, which breaks dynamic ftrace as the address of
ftrace_call is replaced with the address of ftrace_call.cfi_jt. Use
function_nocfi() to get the address of the actual function instead.

Suggested-by: Ben Dai <ben.dai@unisoc.com>
Signed-off-by: Sami Tolvanen <samitolvanen@google.com>
Acked-by: Mark Rutland <mark.rutland@arm.com>
Tested-by: Nathan Chancellor <nathan@kernel.org>
Signed-off-by: Kees Cook <keescook@chromium.org>
Link: https://lore.kernel.org/r/20210408182843.1754385-17-samitolvanen@google.com