MIPS: lib: strnlen_user: Use macro to build the strnlen_user symbol

Build the __strnlen_user symbol using a macro. In EVA mode we will
need to use similar code to do the userspace load operations so
it is better if we use a macro to avoid code duplications.

Signed-off-by: Markos Chandras <markos.chandras@imgtec.com>