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)
Abort the build early if pkg-config dependencies are missing
Get pkg-config dependencies from OPAM
Merge pull request #247 from talex5/arm-preview

ARM support