Deal with float usage

[Nakrez]

Currently floats are not disabled by the target or the build system. The libcore used is not patched to remove float support. This might result in LLVM generating instructions using floats (optimization, ...) which will lead to a crash. This will happen because float related support is not present for any targets.

These two options are available:

• Enable floats on targets that support them and save corresponding registers on context switch / interruptions
• Disable float support from Uni.rs

The second one should probably be the default, but the first one could be activated when the target supports floats (See: rust-lang/rfcs#1364).