macos: link static libc++ when building stage2
macos: link static libc++ when building stage2

With this change, it is now possible to use zig-bootstrap as the
initial C/C++ compiler, and then build stage2 with the resultant
stage1 without the need to link any system libs!

zig-lang/zig,andrewrk/zig,andrewrk/zig,zig-lang/zig,zig-lang/zig