Create __uClibc_main to handle what can be done in C instead of each arch's

Create __uClibc_main to handle what can be done in C instead of each arch's
respective crt0.S.  crt0.S should now only be responsible for setting things
up to call __uClibc_main(argc, argv, envp), which will do any other necessary
setup (setting global __environ, stdio init, etc), call main, and exit.  This
should ease both maintainance and porting.