Use -mno-implicit-float by default for kernel/kext code. <rdar://13177960>

Apple's kernel engineers have been expecting this behavior even though
we've never implemented it before, as far as I can tell. In recent months,
clang has gotten better at using vector instructions to optimize memcpy-like
operations, and that has exposed problems when vector/floating-point
instructions are used in kexts that don't support that. This behavior also
matches what Apple's GCC did for PowerPC targets.

git-svn-id: https://llvm.org/svn/llvm-project/cfe/trunk@174838 91177308-0d34-0410-b5e6-96231b3b80d8