Toolchain with ldc and AArch64 OSX

[Danilo]

Forgot the following flags:
`-L=-merge_zero_fill_sections -L=-no_exported_symbols 
-L=-no_eh_labels -L=-dead_strip_dylibs`

So the full command is:
```
ldc2 --release --O3 --flto=full -fvisibility=hidden 
-defaultlib=phobos2-ldc-lto,druntime-ldc-lto -L=-dead_strip -L=-x 
-L=-S -L=-lz -L=-merge_zero_fill_sections -L=-no_exported_symbols 
-L=-no_eh_labels -L=-dead_strip_dylibs main.d
```
resulting in a executable of 588KB.