use clang as the default backend on darwin

Clang is the default C compiler on darwin. This addres llvm bug 18658 for unused gotos and avoids any issues since a workaround exists in the codebase.

Author: Markos-Th09

src/compiler/fb.bi

@@ -483,7 +483,9 @@ const FB_DEFAULT_CPUTYPE_ASMJS   = FB_CPUTYPE_ASMJS
 	#error Unsupported CPU arch
 #endif
 
-#if defined( __FB_64BIT__ ) or (not defined( __FB_X86__ ))
+#if defined( __FB_DARWIN__ )
+	const FB_DEFAULT_BACKEND = FB_BACKEND_CLANG
+#elseif defined( __FB_64BIT__ ) or (not defined( __FB_X86__ ))
 	const FB_DEFAULT_BACKEND = FB_BACKEND_GCC
 #else
 	const FB_DEFAULT_BACKEND = FB_BACKEND_GAS

src/compiler/fbc.bas

@@ -3062,6 +3062,9 @@ private sub hCheckArgs()
 	if( (fbGetCpuFamily( ) = FB_CPUFAMILY_X86) and _
 		(fbGetOption(FB_COMPOPT_TARGET) <> FB_COMPTARGET_DARWIN) ) then
 		fbSetOption( FB_COMPOPT_BACKEND, FB_BACKEND_GAS )
+	elseif (fbGetOption( FB_COMPOPT_TARGET ) = FB_COMPTARGET_DARWIN) then
+		'' Use clang backend by default on darwin
+		fbSetOption( FB_COMPOPT_BACKEND, FB_BACKEND_CLANG )
 	else
 		fbSetOption( FB_COMPOPT_BACKEND, FB_BACKEND_GCC )
 	end if