Merge pull request #8 from talex5/master

Update to minios-v0.4.2
Build Openlibm with Mini-OS CFLAGS

In particular, use -mno-red-zone on x86_64.
Merge pull request #10 from talex5/xen_cflags

Build Openlibm with Mini-OS CFLAGS
Only build libopenlibm.a

Running the tests doesn't make sense, as we're buildin for Mini-OS.
Travis: test we can build mirage-xen