cross-GDC for tiny embedded Linux

[Dmitry Ponyatov]

> How/where are you referencing mmap64? Could some include be 
> missing or path issue?

ref/gcc-12.3.0/libphobos/libdruntime/core/internal/gc/os.d:145
ref/gcc-12.3.0/libphobos/libdruntime/core/thread/fiber.d:1050
host/lib/gcc/i686-linux-musl/12.3.0/../../../../i686-linux-musl/lib/libgphobos.a(os.o)
etc