LibC: Tidy up _start a bit and rename compilation unit to "crt0"

LibC/entry.cpp

@@ -1,40 +0,0 @@
-#include <stdio.h>
-#include <string.h>
-#include <Kernel/Syscall.h>
-#include <AK/StringImpl.h>
-
-extern "C" {
-
-int main(int, char**);
-
-int errno;
-char** environ;
-
-void __malloc_init();
-void __stdio_init();
-
-int _start(int argc, char** argv, char** env)
-{
-    errno = 0;
-    environ = env;
-
-    __stdio_init();
-    __malloc_init();
-
-    int status = main(argc, argv);
-
-    fflush(stdout);
-    fflush(stderr);
-
-    syscall(SC_exit, status);
-
-    // Birger's birthday <3
-    return 20150614;
-}
-
-[[noreturn]] void __cxa_pure_virtual()
-{
-    ASSERT_NOT_REACHED();
-}
-
-}