[libc] Pull crt0 out into its own module

Sorta. crt0 is a separate module as far as bonnibel is concerned, but it's
still part of the libc module file.

assets/build/target.user.exe.yaml

@@ -1,4 +1,5 @@
 ---
+
 ccflags: [
   "-fpie"
 ]
@@ -8,3 +9,7 @@ ldflags: [
   "--dynamic-linker", "/jsix/lib/ld.so",
   "--push-state", "--as-needed", "-Bstatic", "-lc++", "-lc++abi", "-lunwind", "--pop-state",
 ]
+
+libs: [
+  "${target_dir}/crt0.o",
+]