MIPS: lib: strnlen_user: Add EVA support

In non-EVA mode, a strlen_user* alias is used for the
strlen_kernel* symbols since the code is identical. In EVA
mode, a new strlen_user* symbol is used which uses the EVA
specific instructions to load values from userspace.

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