* Allow skipping parts of the installation to be able to do it in parts if one

* Allow skipping parts of the installation to be able to do it in parts if one
  phase fails and the user wants to reinstall one of the components.
* Fixed LLVM-GCC configuration flags: s/--with-gnu-{as,ld}/--with-{as,ld}/
  The former is a boolean flag, the latter is a flag that takes a path.
* Added a new flag CROSS_MARCH, defaults to armv6.

git-svn-id: https://llvm.org/svn/llvm-project/llvm/trunk@74840 91177308-0d34-0410-b5e6-96231b3b80d8