Add gmp 6.0.0
Configure (esp. config.guess) gets confused by an arm dualcore
Imported zarith from opam-repository
Depend on gmp, not conf-gmp

We need to recompile gmp specifically for Mirage.
Add OCaml compilers for Xen/Mini-OS

Sets CFLAGS and LDFLAGS to find C libraries and doesn't build various
unnecessary bits (e.g. graphics).

Passes -fno-stack-protector -mno-red-zone -DMINIOS -m64 flags on x86_64
to gcc.