Fix: Make VISIBLE expand to nothing for Clang.

The `VISIBLE` macro is designed to selectively inhibit the effects of GCC's
whole program and link-time optimisations. This is necessary when a C function
is only referenced from a context outside the compiler's visibility, e.g. an
assembly file. As far as I can determine, Clang's link-time optimisations
already account for this possibility and do not need to have this information
manually indicated to them (see, for example, Linux's compiler support headers).

In any event, `__attribute__((visibility("default")))` is not equivalent to
`__attribute__((externally_visible))`, but is instead for controlling symbol
visibility in a library-like setting. This commit removes this incorrect
expansion.

include/util.h

@@ -29,7 +29,7 @@
 #define ALIGN(n)     __attribute__((__aligned__(n)))
 #define FASTCALL     __attribute__((fastcall))
 #ifdef __clang__
-#define VISIBLE      __attribute__((visibility("default")))
+#define VISIBLE      /* nothing */
 #else
 #define VISIBLE      __attribute__((externally_visible))
 #endif