build: Support platforms without symbol versioning

The .symver directive is ELF-specific. On non-ELF platforms, work around
this with __attribute__((__alias__)) for the default symbol, and ignore
the variant versioned symbols.
Based-on-patch-by: Aaron Dierking <aarond@fb.com>
Signed-off-by: Guillem Jover <guillem@hadrons.org>

src/local-link.h

@@ -31,10 +31,17 @@
 	static const char libbsd_emit_link_warning_##symbol[] \
 		__attribute__((__used__,__section__(".gnu.warning." #symbol))) = msg;
 
+#ifdef __ELF__
 #define libbsd_symver_default(alias, symbol, version) \
 	__asm__(".symver " #symbol "," #alias "@@" #version)
 
 #define libbsd_symver_variant(alias, symbol, version) \
 	__asm__(".symver " #symbol "," #alias "@" #version)
+#else
+#define libbsd_symver_default(alias, symbol, version) \
+	extern __typeof(symbol) alias __attribute__((__alias__(#symbol)))
+
+#define libbsd_symver_variant(alias, symbol, version)
+#endif
 
 #endif