behavior: skip newly failing test

This test has regressed due to a bug in the self-hosted COFF linker.
Jakub recommended disabling it for now, since the COFF linker is being
effectively rewritten, so there is little point in fixing the bug now.

Author: mlugg

test/behavior/extern.zig

@@ -5,6 +5,7 @@ const expect = std.testing.expect;
 test "anyopaque extern symbol" {
     if (builtin.zig_backend == .stage2_c) return error.SkipZigTest;
     if (builtin.zig_backend == .stage2_wasm) return error.SkipZigTest;
+    if (builtin.zig_backend == .stage2_x86_64 and builtin.target.ofmt != .elf and builtin.target.ofmt != .macho) return error.SkipZigTest;
     if (builtin.zig_backend == .stage2_spirv64) return error.SkipZigTest;
 
     const a = @extern(*anyopaque, .{ .name = "a_mystery_symbol" });