Revert "ARM: 9144/1: forbid ftrace with clang and thumb2_kernel"

This reverts commit ecb108e3.

Clang + Thumb2 with ftrace is now supported.

Signed-off-by: Ard Biesheuvel <ardb@kernel.org>
Reviewed-by: Nick Desaulniers <ndesaulniers@google.com>
Reviewed-by: Steven Rostedt (Google) <rostedt@goodmis.org>