Add more compiler-rt functions for ARM platform

[LemonBoy]

Reported by @via on IRC.

Until #3977 is merged using any fp-related builtin is prone to failure if VFP is enabled.

[frmdstryr]

Thanks, just hit this yesterday and this fixes the issue!

The file compiler_rt/floatundisf.zig (ie floatundisf.c) doesn't exist. Can that also be ported while you're at it?

[LemonBoy]

The file compiler_rt/floatundisf.zig (ie floatundisf.c) doesn't exist. Can that also be ported while you're at it?

I had to push the same commit three times because I kept forgetting to git add the new files heh