On Darwin use assembler to support symbol aliases

The alias attribute is not supported on Darwin, but we can alias using
assembler directives instead.

At the same time we switch the non-ELF fallback implementation for
libmd_symver_default to use libmd_alias.

src/local-link.h

@@ -36,14 +36,20 @@
  * is too cumbersome, as it does not work when static linking, and when
  * dynamic linking it does not make the aliases visible within the DLL itself.
  *
- * On macOS we cannot use proper strong aliases.
+ * On macOS we need to use an assembler alias as it does not support
+ * the alias attribute.
  *
  * Instead we use normal function wrapper in those cases, which are way more
  * maintainable.
  */
-#if !defined(_MSC_VER) && !defined(__APPLE__)
+#if defined(__APPLE__)
 #define libmd_alias(alias, symbol) \
-	extern __typeof(symbol) alias __attribute__((__alias__(#symbol)))
+	__asm__(".globl _" #alias); \
+	__asm__(".set _" #alias ", _" #symbol); \
+	extern __typeof__(symbol) alias
+#elif !defined(_MSC_VER)
+#define libmd_alias(alias, symbol) \
+	extern __typeof__(symbol) alias __attribute__((__alias__(#symbol)))
 #endif
 
 #ifdef __ELF__
@@ -54,7 +60,7 @@
 	__asm__(".symver " #symbol "," #alias "@" #version)
 #else
 #define libmd_symver_default(alias, symbol, version) \
-	extern __typeof(symbol) alias __attribute__((__alias__(#symbol)))
+	libmd_alias(alias, symbol)
 
 #define libmd_symver_variant(alias, symbol, version)
 #endif