Fix a new dead_code warning. (#981)

* Fix a new `dead_code` warning.

* Fix breakage caused by `CLOCK_BOOTTIME` being removed from DragonFly.

rust-lang/libc#3509 removed `CLOCK_BOOTIME` from DragonFly, which broke
rustix.

src/backend/libc/time/syscalls.rs

@@ -167,8 +167,8 @@ pub(crate) fn clock_gettime_dynamic(id: DynamicClockId<'_>) -> io::Result<Timesp
         DynamicClockId::Tai => c::CLOCK_TAI,
 
         #[cfg(any(
-            freebsdlike,
             linux_kernel,
+            target_os = "freebsd",
             target_os = "fuchsia",
             target_os = "openbsd"
         ))]

src/backend/linux_raw/reg.rs

@@ -45,6 +45,7 @@ pub(super) trait FromAsm: private::Sealed {
 /// pointer types. They need a type to point to, so we define a custom private
 /// type, to prevent it from being used for anything else.
 #[repr(transparent)]
+#[allow(dead_code)]
 pub(super) struct Opaque(c::c_void);
 
 // Argument numbers.

src/clockid.rs

@@ -149,8 +149,8 @@ pub enum DynamicClockId<'a> {
 
     /// `CLOCK_BOOTTIME`
     #[cfg(any(
-        freebsdlike,
         linux_kernel,
+        target_os = "freebsd",
         target_os = "fuchsia",
         target_os = "openbsd"
     ))]

tests/time/clocks.rs

@@ -8,8 +8,8 @@ use rustix::time::{clock_gettime, ClockId};
 /// Attempt to test that the boot clock is monotonic. Time may or may not
 /// advance, but it shouldn't regress.
 #[cfg(any(
-    freebsdlike,
     linux_kernel,
+    target_os = "freebsd",
     target_os = "fuchsia",
     target_os = "openbsd"
 ))]