MC: Improve the .fill directive's compatibility with GAS

Per the GAS documentation, .fill should permit pattern widths that
aren't a power of two. While I was in the neighborhood, I added some
sanity checking. This change was motivated by a use of this construct
in the Linux Kernel.


git-svn-id: https://llvm.org/svn/llvm-project/llvm/trunk@200606 91177308-0d34-0410-b5e6-96231b3b80d8