Here are what the files do for you. They are the c-runtime environment and setup that link to the OS.
crt1.o
Newer style of the initial runtime code. Contains the _start symbol which sets up the env with argc/argv/libc _init/libc _fini before jumping to the libc main. glibc calls this file 'start.S'.crti.o
Defines the function prolog; _init in the .init section and _fini in the .fini section. glibc calls this 'initfini.c'.crtn.o
Defines the function epilog. glibc calls this 'initfini.c'.
There is an excellent write up and example code to be found at the following website http://wiki.osdev.org/Creating_a_C_Library for each of the libraries above.