These flags were supposed to be relevant for musl, not for gnu

cc rust-lang#39979

This contains two important bugfixes

I made these the same order as they were in the compiler, but for no good reason. Much easier to find out what you need when they're sorted alphabetically

…matsakis

travis: Fix typos in linux-tested-targets

These flags were supposed to be relevant for musl, not for gnu

cc rust-lang#39979

Structs doc: Change "pointers" to "references"

Let's call them "references" instead of "pointers". That's how they're called in chapter 4.9 "References and Borrowing".

r? @steveklabnik

…arget-has-atomic, r=alexcrichton

Add compile test for cfg_target_has_atomic

Issue rust-lang#39059.
I am concerned about whether the test is excessive.

update mdbook version

This contains two important bugfixes

…ook, r=frewsxcv

sort unstable book alphabetically

I made these the same order as they were in the compiler, but for no good reason. Much easier to find out what you need when they're sorted alphabetically

r? @frewsxcv