[fortran,patch] Support for IEEE underflow control on x86/x86_64

[FX]

Hi all,

The attached patch provides support for underflow control in the 
IEEE_ARITHMETIC module, for x86/x86_64 targets (our main user base).
Bootstrapped and regtested on x86_64-apple-darwin13. Comes with a testcase.

OK to commit?

FX

underflow.ChangeLog
Description: Binary data

underflow.diff
Description: Binary data