diff --git a/builtins-test/build.rs b/builtins-test/build.rs
index 5b2dcd12..b14a822b 100644
--- a/builtins-test/build.rs
+++ b/builtins-test/build.rs
@@ -69,6 +69,8 @@ fn main() {
         || target.arch == "powerpc64"
         || target.arch == "powerpc64le"
         || target.arch == "loongarch64"
+        // ABI only recently got specified
+        || target.arch == "s390x"
         || (target.arch == "x86" && !target.has_feature("sse"))
         || target.os == "windows"
         // Linking says "error: function signature mismatch: __extendhfsf2" and seems to
diff --git a/builtins-test/src/lib.rs b/builtins-test/src/lib.rs
index f1673133..09e8c6db 100644
--- a/builtins-test/src/lib.rs
+++ b/builtins-test/src/lib.rs
@@ -15,6 +15,7 @@
 #![no_std]
 #![cfg_attr(f128_enabled, feature(f128))]
 #![cfg_attr(f16_enabled, feature(f16))]
+#![feature(linkage)]
 
 pub mod bench;
 extern crate alloc;
@@ -403,3 +404,30 @@ macro_rules! apfloat_fallback {
         $apfloat_op($($arg),+)
     }};
 }
+
+mod bootstrap {
+    // Needed for testing other symbols,
+    #[cfg(f16_enabled)]
+    #[cfg(target_arch = "s390x")]
+    #[linkage = "weak"]
+    #[unsafe(no_mangle)]
+    pub extern "C" fn __extendhfsf2(a: f16) -> f32 {
+        compiler_builtins::float::extend::__extendhfsf2(a)
+    }
+
+    #[cfg(f128_enabled)]
+    #[cfg(target_arch = "x86")]
+    #[linkage = "weak"]
+    #[unsafe(no_mangle)]
+    pub extern "C" fn __extenddftf2(a: f64) -> f128 {
+        compiler_builtins::float::extend::__extenddftf2(a)
+    }
+
+    #[cfg(f128_enabled)]
+    #[cfg(target_arch = "x86")]
+    #[linkage = "weak"]
+    #[unsafe(no_mangle)]
+    pub extern "C" fn __floattitf(i: i128) -> f128 {
+        compiler_builtins::float::conv::__floattitf(i)
+    }
+}
diff --git a/compiler-builtins/configure.rs b/compiler-builtins/configure.rs
index 79e238ab..5d8744fc 100644
--- a/compiler-builtins/configure.rs
+++ b/compiler-builtins/configure.rs
@@ -55,8 +55,10 @@ impl Target {
                 .collect(),
             // Note that these are unstable options, so only show up with the nightly compiler or
             // with `RUSTC_BOOTSTRAP=1` (which is required to use the types anyway).
-            reliable_f128: env::var_os("CARGO_CFG_TARGET_HAS_RELIABLE_F128").is_some(),
-            reliable_f16: env::var_os("CARGO_CFG_TARGET_HAS_RELIABLE_F16").is_some(),
+            // reliable_f128: env::var_os("CARGO_CFG_TARGET_HAS_RELIABLE_F128").is_some(),
+            // reliable_f16: env::var_os("CARGO_CFG_TARGET_HAS_RELIABLE_F16").is_some(),
+            reliable_f16: true,
+            reliable_f128: true,
         }
     }
 
diff --git a/libm/configure.rs b/libm/configure.rs
index 76186e63..44dee856 100644
--- a/libm/configure.rs
+++ b/libm/configure.rs
@@ -33,13 +33,18 @@ impl Config {
             .map(|s| s.to_lowercase().replace("_", "-"))
             .collect();
 
+        let target_arch = env::var("CARGO_CFG_TARGET_ARCH").unwrap();
+        // Symbols are missing so we can't test until they get to nightly
+        let bootstrapping_f16 = target_arch == "s390x";
+        let bootstrapping_f128 = target_arch == "x86";
+
         Self {
             target_triple,
             manifest_dir: PathBuf::from(env::var("CARGO_MANIFEST_DIR").unwrap()),
             out_dir: PathBuf::from(env::var("OUT_DIR").unwrap()),
             opt_level: env::var("OPT_LEVEL").unwrap(),
             cargo_features,
-            target_arch: env::var("CARGO_CFG_TARGET_ARCH").unwrap(),
+            target_arch,
             target_env: env::var("CARGO_CFG_TARGET_ENV").unwrap(),
             target_family: env::var("CARGO_CFG_TARGET_FAMILY").ok(),
             target_os: env::var("CARGO_CFG_TARGET_OS").unwrap(),
@@ -48,8 +53,10 @@ impl Config {
             target_features,
             // Note that these are unstable options, so only show up with the nightly compiler or
             // with `RUSTC_BOOTSTRAP=1` (which is required to use the types anyway).
-            reliable_f128: env::var_os("CARGO_CFG_TARGET_HAS_RELIABLE_F128").is_some(),
-            reliable_f16: env::var_os("CARGO_CFG_TARGET_HAS_RELIABLE_F16").is_some(),
+            reliable_f16: env::var_os("CARGO_CFG_TARGET_HAS_RELIABLE_F16").is_some()
+                && !bootstrapping_f16,
+            reliable_f128: env::var_os("CARGO_CFG_TARGET_HAS_RELIABLE_F128").is_some()
+                && !bootstrapping_f128,
         }
     }
 }