[linker] Add safe weak symbol macros

Weak symbols are a useful tool in eliminating unnecessary dependencies
between object files, but they are somewhat dangerous because one must
remember to test the weak symbol against NULL before using it. To
rectify that, add macros for declaring weak functions that will return
a default value inline if the file defining them is not available at
link time.

Signed-off-by: Marty Connor <mdc@etherboot.org>

src/include/compiler.h

 /** Select file identifier for errno.h (if used) */
 #define ERRFILE PREFIX_OBJECT ( ERRFILE_ )
 
+/**
+ * @defgroup weakmacros Macros to manage weak symbol definitions
+ *
+ * Weak symbols allow one to reference a function in another file
+ * without necessarily requiring that file to be linked in. In their
+ * native form, the function will be @c NULL if its file is not linked
+ * in; these macros provide an inline wrapper that returns an
+ * appropriate error indication or default value.
+ *
+ * @{
+ */
+#ifndef ASSEMBLY
+
+/** Mangle @a name into its weakly-referenced implementation */
+#define __weak_impl( name )   _w_ ## name
+
+/**
+ * Declare a weak function with inline safety wrapper
+ *
+ * @v ret	Return type of weak function
+ * @v name	Name of function to expose
+ * @v proto	Parenthesized list of arguments with types
+ * @v args	Parenthesized list of argument names
+ * @v dfl	Value to return if weak function is not available
+ */
+#define __weak_decl( ret, name, proto, args, dfl )		\
+        ret __weak_impl( name ) proto __attribute__ (( weak ));	\
+        static inline ret name proto {				\
+                if ( __weak_impl( name ) )			\
+                        return __weak_impl( name ) args;	\
+                return dfl;					\
+        }
+
+#endif
+/** @} */
+
 /** @defgroup dbg Debugging infrastructure
  * @{
  */