Raise libc's FreeBSD ABI to 12

FreeBSD 11 will be EoL on 30-Sept-2021.  Update libc's ABI to the
minimum supported version of FreeBSD.

build.rs

@@ -20,7 +20,7 @@ fn main() {
     }
 
     // The ABI of libc used by libstd is backward compatible with FreeBSD 10.
-    // The ABI of libc from crates.io is backward compatible with FreeBSD 11.
+    // The ABI of libc from crates.io is backward compatible with FreeBSD 12.
     //
     // On CI, we detect the actual FreeBSD version and match its ABI exactly,
     // running tests to ensure that the ABI is correct.
@@ -31,7 +31,7 @@ fn main() {
         Some(11) if libc_ci => println!("cargo:rustc-cfg=freebsd11"),
         Some(12) if libc_ci => println!("cargo:rustc-cfg=freebsd12"),
         Some(13) if libc_ci => println!("cargo:rustc-cfg=freebsd13"),
-        Some(_) | None => println!("cargo:rustc-cfg=freebsd11"),
+        Some(_) | None => println!("cargo:rustc-cfg=freebsd12"),
     }
 
     // On CI: deny all warnings