[build] Remove unneeded lib options

Remove the `-lc++`, `-lc++abi`, and `-lunwind` options from the user
target, where they should be handled automatically. (ie, we're not using
`-nostdlib` or its bretheren.)

assets/build/target.user.exe.yaml

@@ -6,4 +6,5 @@ ccflags: [
 ldflags: [
   "-pie",
   "--dynamic-linker", "/tools/ld.so",
+  "-lc++", "-lc++abi", "-lunwind",
 ]