Pass --target in cargo.sh

This reverts commit 590bfc6. The
target name is now supplied by `$TARGET_SPEC` instead of
`$TARGET_TRIPLE` to support target spec files.

Author: yvt

cargo.sh

@@ -20,4 +20,4 @@ fi
 cmd=$1
 shift
 
-RUSTDOCFLAGS="$RUSTFLAGS" cargo +${TOOLCHAIN} $cmd $@
+RUSTDOCFLAGS="$RUSTFLAGS" cargo +${TOOLCHAIN} $cmd --target "$TARGET_SPEC" $@