mk: make NO_REBUILD more forceful and more general.

Previously crates like `green` and `native` would still depend on their
parents when running `make check-stage2-green NO_REBUILD=1`, this
ensures that they only depend on their source files.

Also, apply NO_REBUILD to the crate doc tests, so, for example,
`check-stage2-doc-std` will use an already compiled `rustdoc` directly.
auto merge of #12237 : huonw/rust/norebuild++, r=alexcrichton

Previously crates like `green` and `native` would still depend on their
parents when running `make check-stage2-green NO_REBUILD=1`, this
ensures that they only depend on their source files.

Also, apply NO_REBUILD to the crate doc tests, so, for example,
`check-stage2-doc-std` will use an already compiled `rustdoc` directly.