ARM: breakpoint: CFI breakpoints only on demand

This removes the stub hw_breakpoint_cfi_handler() from ARM, making
it not steal breakpoint type 0x03 (ARM_ENTRY_CFI_BREAKPOINT) unless
CFI is actively used in the kernel.

When not instrumenting with CFI, or when a breakpoint is issued in
userspace, we fall through to return 1 from hw_breakpoint_pending()
"unhandled fault" so userspace can make use of this breakpoint.

Tested with LKDTM and this command line:
echo CFI_FORWARD_PROTO > /sys/kernel/debug/provoke-crash/DIRECT
still works as expected.

Fixes: c3f89986fde7 ("ARM: 9391/2: hw_breakpoint: Handle CFI breakpoints")
Reported-by: slipher <slipher@protonmail.com>
Suggested-by: Mark Rutland <mark.rutland@arm.com>
Closes: https://lore.kernel.org/lkml/kJqktbpLphg_Pk5I5SPptgTLjl3E3eq5mN5UzCslyFj7Q1Irp-wDid4mj5eQVd2iZtRGXgeZd8goq195EkXdjyt864YMc8mVb2B9NGH91NQ=@protonmail.com/
Signed-off-by: Linus Walleij <linusw@kernel.org>