tools: repair Windows build

exp2 is not available on Windows.  Fortunately, we are calculating powers of 2
with expontents within the range of [4,12].  Simply use an equivalent bitshift
operation to repair compilation with MSVC which does not provide this standard
function.

git-svn-id: https://llvm.org/svn/llvm-project/llvm/trunk@200454 91177308-0d34-0410-b5e6-96231b3b80d8