system/glue: Add a crtbegin and crtend implementation.

GCC provides these but Clang does not; these come from the ones Clang
was going to provide, but decided against it in favor of letting libc vendors
distribute them instead, hence the license headers.

For now, they are only enabled when building with Clang, as we still use GCC's
versions otherwise. But I've tested them with apps built with GCC 7 (as Clang
builds are still too broken at present) and they seem to work just fine. (They
will not build with GCC 2 of course, due to how they use __attribute__.)