Format libstd/sys with rustfmt

This commit applies rustfmt with rust-lang/rust's default settings to
files in src/libstd/sys *that are not involved in any currently open PR*
to minimize merge conflicts. THe list of files involved in open PRs was
determined by querying GitHub's GraphQL API with this script:
https://gist.github.com/dtolnay/aa9c34993dc051a4f344d1b10e4487e8

With the list of files from the script in outstanding_files, the
relevant commands were:

    $ find src/libstd/sys -name '*.rs' \
        | xargs rustfmt --edition=2018 --unstable-features --skip-children
    $ rg libstd/sys outstanding_files | xargs git checkout --

Repeating this process several months apart should get us coverage of
most of the rest of the files.

To confirm no funny business:

    $ git checkout $THIS_COMMIT^
    $ git show --pretty= --name-only $THIS_COMMIT \
        | xargs rustfmt --edition=2018 --unstable-features --skip-children
    $ git diff $THIS_COMMIT  # there should be no difference

1 files changed, 2 insertions, 2 deletions

diff --git a/src/libstd/sys/unix/cmath.rs b/src/libstd/sys/unix/cmath.rs
index f6bb58934fc..2916ebe4440 100644
--- a/src/libstd/sys/unix/cmath.rs
+++ b/src/libstd/sys/unix/cmath.rs
@@ -1,9 +1,9 @@
 #![cfg(not(test))]
 
-use libc::{c_float, c_double};
+use libc::{c_double, c_float};
 
 #[link_name = "m"]
-extern {
+extern "C" {
     pub fn acos(n: c_double) -> c_double;
     pub fn acosf(n: c_float) -> c_float;
     pub fn asin(n: c_double) -> c_double;