Fix #170: Move get-system-info to C
- Define 
os_get_system_infoin os-common.c to return the desired system info. - Add 
get-system-infoto unix.lisp to call os_get_system_info. - Remove 
get-system-infofrom linux-os.lisp 
The return type of get-system-info is slightly changed
to `(values (signed-byte 64) (signed-byte 64) (signed-byte 32))