RSB LLVM Package

[chrisj at rtems.org]

Hello,

These patches add LLVM as a package to rtems-5. You can build LLVM to a prefix
using ...

 ../source-builder/sb-setbuilder --prefix=/some/where --log=llvm.txt 5/rtems-llvm

- The package builds swig as it is used by lldb. The swig package internally
  builds and statically links PCRE. There are some changes to the RSB build
  code to handle downloading but not installing a source package.

- LLVM lldb is not built on MacOS by default. You can override the default
  by adding --with-llvm-lldb. If you do this you will need to have a 
  self signed codesign certificate in your keyring named 'llvm' as lldb
  has to be codesigned in order to run on MacOS.

- I have tested building on FreeBSD and MacOS and would welcome Linux and MSYS2
  feedback.

- There is currently no integration with rtems-tools trace support which
  is now using LLVM. It would nice to have this added so the tools built
  are common for all hosts.

- The LLVM compiler is tagged with our RSB details. All targets are built.
  I am not sure how we build newlib or how we build RTEMS. Suggestions or
  patches? :)

Chris