riscv: Stop supporting static ftrace

riscv: Stop supporting static ftrace

Now that DYNAMIC_FTRACE was introduced, there is no need to support
static ftrace as it is way less performant. This simplifies the code and
prevents build failures as reported by kernel test robot when
!DYNAMIC_FTRACE.

Also make sure that FUNCTION_TRACER can only be selected if
DYNAMIC_FTRACE is supported (we have a dependency on the toolchain).

Co-developed-by: chenmiao <chenmiao.ku@gmail.com>
Signed-off-by: chenmiao <chenmiao.ku@gmail.com>
Fixes: b2137c3b6d7a ("riscv: ftrace: prepare ftrace for atomic code patching")
Reported-by: kernel test robot <lkp@intel.com>
Closes: https://lore.kernel.org/oe-kbuild-all/202506191949.o3SMu8Zn-lkp@intel.com/
Signed-off-by: Alexandre Ghiti <alexghiti@rivosinc.com>
Link: https://lore.kernel.org/r/20250716-dev-alex-static_ftrace-v1-1-ba5d2b6fc9c0@rivosinc.com
Signed-off-by: Palmer Dabbelt <palmer@dabbelt.com>