Recognize -mno-implicit-float option for x86 as well as ARM. <rdar://13180731>

For x86 targets, we've been using the -msoft-float option to control passing
the no-implicit-float option to cc1. Since the -mno-implicit-float option is
now accepted by the driver, this just makes it work for x86 the same as it
does for ARM targets.

git-svn-id: http://llvm.org/svn/llvm-project/cfe/trunk@174836 91177308-0d34-0410-b5e6-96231b3b80d8