MIPS: lib: strlen_user: Add EVA support

In non-EVA mode, strlen_user* aliases are used for the
strlen_kernel* symbols since the code is identical. In EVA
mode, new strlen_user* symbols are used which use the EVA
specific instructions to load values from userspace.

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