Introduce MBEDTLS_COMPILER_IS_GCC

Signed-off-by: Dave Rodgman <dave.rodgman@arm.com>

library/bn_mul.h

@@ -661,14 +661,10 @@
 #if defined(__arm__)
 
 #if defined(__thumb__) && !defined(__thumb2__)
-#if !defined(__ARMCC_VERSION) && !defined(__clang__) \
-    && !defined(__llvm__) && !defined(__INTEL_COMPILER)
+#if defined(MBEDTLS_COMPILER_IS_GCC)
 /*
  * 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.
  */
 
 #if !defined(__OPTIMIZE__) && defined(__GNUC__)

library/common.h

@@ -195,4 +195,10 @@ inline void mbedtls_xor(unsigned char *r, const unsigned char *a, const unsigned
 #define MBEDTLS_UNLIKELY(x)     x
 #endif
 
+#if defined(__GNUC__) && !defined(__ARMCC_VERSION) && !defined(__clang__) \
+    && !defined(__llvm__) && !defined(__INTEL_COMPILER)
+/* Defined if the compiler really is gcc and not clang, etc */
+#define MBEDTLS_COMPILER_IS_GCC
+#endif
+
 #endif /* MBEDTLS_LIBRARY_COMMON_H */