mips32: finetune vfp test to avoid compiler warnings

mips32: finetune vfp test to avoid compiler warnings

This patch removes two compiler warnings from the test:

vfp.c: In function 'handler':
vfp.c:260:4: warning: implicit declaration of function 'exit'
[-Wimplicit-function-declaration]
    exit(0);
    ^
vfp.c:260:4: warning: incompatible implicit declaration of built-in
function 'exit'
vfp.c: At top level:
vfp.c:258:13: warning: 'handler' defined but not used [-Wunused-function]
 static void handler(int sig)
             ^