Fix nostd build

cgemm was not tested as nostd in ci

ensure_no_std/Cargo.toml

@@ -6,7 +6,7 @@ edition = "2018"
 publish = false
 
 [dependencies]
-matrixmultiply = { path = "..", default-features = false }
+matrixmultiply = { path = "..", default-features = false, features = ["cgemm"] }
 
 [profile.dev]
 panic = "abort"

src/cgemm_common.rs

@@ -6,14 +6,15 @@
 // option. This file may not be copied, modified, or distributed
 // except according to those terms.
 
-use std::mem;
-use std::ptr::copy_nonoverlapping;
+use core::mem;
+use core::ptr::copy_nonoverlapping;
 
 use rawpointer::PointerExt;
 
 use crate::kernel::Element;
 use crate::kernel::ConstNum;
 
+#[cfg(feature = "std")]
 macro_rules! fmuladd {
     // conceptually $dst += $a * $b, optionally use fused multiply-add
     (fma_yes, $dst:expr, $a:expr, $b:expr) => {
@@ -28,6 +29,15 @@ macro_rules! fmuladd {
     };
 }
 
+#[cfg(not(feature = "std"))]
+macro_rules! fmuladd {
+    ($any:tt, $dst:expr, $a:expr, $b:expr) => {
+        {
+            $dst += $a * $b;
+        }
+    };
+}
+
 
 // kernel fallback impl macro
 // Depends on a couple of macro and function defitions to be in scope - loop_m/_n, at, etc.