Add runtime support for float-invalid exceptions

This exception is thrown by libm functions, so it should
be translated to conditions just like the other ones.
Add HARP support for float-invalid.
[rt] Set correct FP exception mask on FreeBSD.
Merge pull request #642 from dylan-lang/sigfpe-changes

SIGFPE changes -- thanks (according to bruce this survives at least a 3-stage-bootstrap)