Disable link time optimization for microkernel library

* Our SME kernels are compiled with auto-vectorization disabled,
  but the linker is unable to respect this flag, which can
  cause miscompilation and result in illegal instruction.

Signed-off-by: Viet-Hoa Do <viet-hoa.do@arm.com>

Approved-by: Emil Ohlsson <emil.ohlsson@arm.com>