Auto merge of #7257 - RobMor:no-spaces-between-flags, r=alexcrichton

Auto merge of #7257 - RobMor:no-spaces-between-flags, r=alexcrichton

Fix #7217: Add support for rustc-flags without spaces between flags and values

Hi, I believe this pull request contains a fix for issue #7217. This is my first pull request to any open source project, much less any Rust project. I'm not super familiar with Rust at the moment, so let me know if I should change anything. Also any help/advice you can give me about this PR would be much appreciated!

I do have some specific questions:
- Is the test I added worthy of being its own test? I basically copied the one above it and remove the spaces between the flags and the values. Should I have added on to the test above it instead?
- Would it be better if I directly unit-tested this function in some other test file?
- Is the `ureachable!` macro good style?