Updated "mirage configure --xen" for new Mini-OS and libm

We now get these via pkg-config.
Link libgcc.a

This is needed on ARM. Without it, you get:

_build/main.native.o: In function `.L104':
camlinternalOO.o:(.text+0xb36c): undefined reference to `__aeabi_dcmpge'
camlinternalOO.o:(.text+0xb3a0): undefined reference to `__aeabi_dadd'
...
Convert from ELF to binary format (for ARM)
Merge pull request #255 from samoht/master

Expose the clock combinator + various clean-ups
add an entropy device for mirage
fix typo here
type t = unit instead of string
rename error constructor
commit to a cstruct.t as buffer
use different module names for unix and xen
Sync changes
Allow the nature of the entropy device to be specified in the device

We classify this into `Strongest` for the best one, and `Strong`
and `Weak` if one is to be specifically selected.  The default is
`Strongest`, which maps to `/dev/random` on Unix and currently a
weak (but functioning) source on Xen.
Merge pull request #259 from avsm/hannesm-add-entropy-device

Updates to entropy device support
Be more explicit about the names of Entropy devices
Revert b872b93d9056b3 to only expose a `default_entropy` for now.

We will eventually need a xen-specific entropy device to specify
the backend configuration (for `rndback`), but for now the module
just self-seeds as best it can.
Merge pull request #260 from avsm/entropy-1.2

Be more explicit about the names of Entropy devices
rework the ENTROPY signature, give evented semantics
Merge pull request #261 from mirage/evented-entropy

Move the ENTROPY sig to evented semantics
change mirage tool to apply Entropy_unix functor
disambiguate module names