CodeGen: Cast llvm.flt.rounds result to match __builtin_flt_rounds

llvm.flt.rounds returns an i32, but the builtin expects an integer. 
On targets where integers are not 32-bits clang tries to bitcast the result, causing an assertion failure.

The patch enables newlib build for msp430.

Patch by Edward Jones!

Differential Revision: https://reviews.llvm.org/D24461

llvm-svn: 351449