Add RUST_STD_FREEBSD_12_ABI env variable

Unfortunately, sanitizers do not support versioned symbols[1],
so they break filesystem access via the legacy, pre-ino64 ABI.

To use sanitizers on FreeBSD >= 12, we need to build the libc
crate with LIBC_CI=1 to use the new ABI -- including the libc
used for std. But that removes the st_lspare field std was
expecting for the deprecated metadata extension.

Add a way to skip that field to allow the build to work.

[1]: google/sanitizers#628

library/std/build.rs

@@ -16,6 +16,9 @@ fn main() {
     } else if target.contains("freebsd") {
         println!("cargo:rustc-link-lib=execinfo");
         println!("cargo:rustc-link-lib=pthread");
+        if env::var("RUST_STD_FREEBSD_12_ABI").is_ok() {
+            println!("cargo:rustc-cfg=freebsd12");
+        }
     } else if target.contains("netbsd") {
         println!("cargo:rustc-link-lib=pthread");
         println!("cargo:rustc-link-lib=rt");

library/std/src/os/freebsd/fs.rs

@@ -74,6 +74,11 @@ pub trait MetadataExt {
 impl MetadataExt for Metadata {
     #[allow(deprecated)]
     fn as_raw_stat(&self) -> &raw::stat {
+        // The methods below use libc::stat, so they work fine when libc is built with FreeBSD 12 ABI.
+        // This method would just return nonsense.
+        #[cfg(freebsd12)]
+        panic!("as_raw_stat not supported with FreeBSD 12 ABI");
+        #[cfg(not(freebsd12))]
         unsafe { &*(self.as_inner().as_inner() as *const libc::stat as *const raw::stat) }
     }
     fn st_dev(&self) -> u64 {
@@ -137,6 +142,9 @@ impl MetadataExt for Metadata {
         self.as_inner().as_inner().st_flags as u32
     }
     fn st_lspare(&self) -> u32 {
+        #[cfg(freebsd12)]
+        panic!("st_lspare not supported with FreeBSD 12 ABI");
+        #[cfg(not(freebsd12))]
         self.as_inner().as_inner().st_lspare as u32
     }
 }