Raise libc's FreeBSD ABI to 12

FreeBSD 11 was EoL on 30-Sept-2021.  Update libc's ABI to 12.  That
version includes significant changes, such as 64-bit inodes.

(backport <#2406>)
(cherry picked from commit 4bafe6b)

Author: asomers

build.rs

@@ -58,7 +58,7 @@ fn main() {
     let target_arch = env::var("CARGO_CFG_TARGET_ARCH").unwrap_or_default();
 
     // The ABI of libc used by std is backward compatible with FreeBSD 12.
-    // 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.
@@ -69,11 +69,11 @@ fn main() {
         println!("cargo:warning=setting FreeBSD version to {vers}");
         vers
     } else if libc_ci {
-        which_freebsd().unwrap_or(11)
+        which_freebsd().unwrap_or(12)
     } else if rustc_dep_of_std {
         12
     } else {
-        11
+        12
     };
 
     match which_freebsd {