Fix armclang compile fail

Signed-off-by: Dave Rodgman <[email protected]>

ChangeLog.d/armclang-compile-fix.txt

@@ -0,0 +1,5 @@
+Bugfix
+   * Fix clang and armclang compilation error when targeting certain Arm
+     M-class CPUs (Cortex-M0, Cortex-M0+, Cortex-M1, Cortex-M23,
+     SecurCore SC000). Fixes #1077.
+

include/mbedtls/bn_mul.h

@@ -677,6 +677,15 @@
 #if defined(__arm__) && !defined(MULADDC_CANNOT_USE_R7)
 
 #if defined(__thumb__) && !defined(__thumb2__)
+#if !defined(__ARMCC_VERSION) && !defined(__clang__) \
+    && !defined(__llvm__) && !defined(__INTEL_COMPILER)
+/*
+ * Thumb 1 ISA. This code path has only been tested successfully on gcc;
+ * it does not compile on clang or armclang.
+ *
+ * Other compilers which define __GNUC__ may not work. The above macro
+ * attempts to exclude these untested compilers.
+ */
 
 #define MULADDC_INIT                                    \
     asm(                                                \
@@ -731,6 +740,8 @@
            "r6", "r7", "r8", "r9", "cc"         \
          );
 
+#endif /* Compiler is gcc */
+
 #elif (__ARM_ARCH >= 6) && \
     defined (__ARM_FEATURE_DSP) && (__ARM_FEATURE_DSP == 1)