mips: Disable attribute-alias warnings

This warning is seen with gcc-8
include/linux/syscalls.h:211:18: error: 'sys_cachectl' alias between functions of incom
patible types 'long int(char *, int,  int)' and 'long int(long int,  long int,  long int)'

Signed-off-by: Khem Raj <raj.khem@gmail.com>
Signed-off-by: Bruce Ashfield <bruce.ashfield@windriver.com>