arm64: ftrace: use function_nocfi for _mcount as well

Commit 800618f ("arm64: ftrace: use function_nocfi for ftrace_call")
only fixed address of ftrace_call but address of _mcount needs to be
fixed as well. Use function_nocfi() to get the actual address of _mcount
function as with CONFIG_CFI_CLANG, the compiler replaces function pointers
with jump table addresses which breaks dynamic ftrace as the address of
_mcount is replaced with the address of _mcount.cfi_jt.

With mainline, this won't be a problem since by default
CONFIG_DYNAMIC_FTRACE_WITH_REGS=y with Clang >= 10 as it supports
-fpatchable-function-entry and CFI requires Clang 12 but for consistency
we should add function_nocfi() for _mcount as well.

Signed-off-by: Sumit Garg <[email protected]>
Acked-by: Mark Rutland <[email protected]>
Reviewed-by: Sami Tolvanen <[email protected]>
Link: https://lore.kernel.org/r/[email protected]
Signed-off-by: Will Deacon <[email protected]>

Author: b49020

arch/arm64/include/asm/ftrace.h

@@ -15,7 +15,7 @@
 #ifdef CONFIG_DYNAMIC_FTRACE_WITH_REGS
 #define ARCH_SUPPORTS_FTRACE_OPS 1
 #else
-#define MCOUNT_ADDR		((unsigned long)_mcount)
+#define MCOUNT_ADDR		((unsigned long)function_nocfi(_mcount))
 #endif
 
 /* The BL at the callsite's adjusted rec->ip */