build: add --with-python option

This adds a new --with-python option to allow configuration of the python binary
for building.  If not specified, $PATH will be searched for common python binary
names (python, python2, python3).  If specified, and the path is not executable,
it will attempt to search $PATH.

Signed-off-by: Saleem Abdulrasool <compnerd@compnerd.org>
Reviewed-by: Eric Christopher <echristo@gmail.com>, Daniel Dunbar <daniel@zuster.org>

git-svn-id: https://llvm.org/svn/llvm-project/llvm/trunk@173890 91177308-0d34-0410-b5e6-96231b3b80d8