Merged compiler-polyfill into toolchain.h

from https://github.com/armmbed/compiler-polyfill

notes
- Adopted existing mbed naming convention of all caps.
  This avoids conflicts with compiler declarations
  https://github.com/ARMmbed/compiler-polyfill/issues/7

- Dropped align attribute due to lack of support on IAR.

- Currently toolchain.h lives in /hal/api, although with
  the addition of /util there may be a better home.

hal/api/toolchain.h

@@ -16,6 +16,107 @@
 #ifndef MBED_TOOLCHAIN_H
 #define MBED_TOOLCHAIN_H
 
+
+// Warning for unsupported compilers
+#if !defined(__GNUC__)   /* GCC        */ \
+ && !defined(__CC_ARM)   /* ARMCC      */ \
+ && !defined(__clang__)  /* LLVM/Clang */ \
+ && !defined(__ICCARM__) /* IAR        */
+#warning "This compiler is not yet supported."
+#endif
+
+
+// Attributes
+
+/** PACK
+ *  Pack a structure, preventing any padding from being added between fields.
+ *
+ *  @code
+ *  #include "toolchain.h"
+ *
+ *  struct foo {
+ *      char x;
+ *      int y;
+ *  } PACKED;
+ *  @endcode
+ */
+#ifndef PACKED
+#if defined(__ICCARM__)
+#define PACKED __packed
+#else
+#define PACKED __attribute__((packed))
+#endif
+#endif
+
+/** UNUSED
+ *  Declare a function argument to be unused, suppressing compiler warnings
+ *
+ *  @code
+ *  #include "toolchain.h"
+ *
+ *  void foo(UNUSED int arg){
+ *
+ *  }
+ *  @endcode
+ */
+#ifndef UNUSED
+#if defined(__GNUC__) || defined(__CC_ARM) || defined(__clang__)
+#define UNUSED __attribute__((__unused__))
+#else
+#define UNUSED
+#endif
+#endif
+
+/** WEAK
+ *  Mark a function as being weak.
+ *  
+ *  @note
+ *  weak functions are not friendly to making code re-usable, as they can only
+ *  be overridden once (and if they are multiply overridden the linker will emit
+ *  no warning). You should not normally use weak symbols as part of the API to
+ *  re-usable modules.
+ *  
+ *  @code
+ *  #include "toolchain.h"
+ *  
+ *  WEAK void foo() {
+ *      // a weak implementation of foo that can be overriden by a definition
+ *      // without  __weak
+ *  }
+ *  @endcode
+ */
+#ifndef WEAK
+#if defined(__ICCARM__)
+#define WEAK __weak
+#else
+#define WEAK __attribute__((weak))
+#endif
+#endif
+
+/** DEPRECATED("message string")
+ *  Mark a function declaration as deprecated, if it used then a warning will be
+ *  issued by the compiler possibly including the provided message. Note that not
+ *  all compilers are able to display the message.
+ *  
+ *  @code
+ *  #include "toolchain.h"
+ *  
+ *  DEPRECATED("don't foo any more, bar instead")
+ *  void foo(int arg);
+ *  @endcode
+ */
+#ifndef DEPRECATED
+#if defined(__GNUC__) || defined(__clang__)
+#define DEPRECATED(M) __attribute__((deprecated(M)))
+#elif defined(__CC_ARM)
+#define DEPRECATED(M) __attribute__((deprecated))
+#else
+#define DEPRECATED(M)
+#endif
+#endif
+
+
+// FILEHANDLE declaration
 #if defined(TOOLCHAIN_ARM)
 #include <rt_sys.h>
 #endif
@@ -24,16 +125,10 @@
 typedef int FILEHANDLE;
 #endif
 
+// Backwards compatibility
 #ifndef EXTERN
 #define EXTERN extern
 #endif
 
-#if defined (__ICCARM__)
-#   define WEAK     __weak
-#   define PACKED   __packed
-#else
-#   define WEAK     __attribute__((weak))
-#   define PACKED   __attribute__((packed))
-#endif
 
 #endif