Update GettingStarted docs list of LLVM_TARGETS_TO_BUILD to match cmake.

Update GettingStarted docs list of LLVM_TARGETS_TO_BUILD to match cmake.

Since the docs were written, we've added the BPF backend to the list.
Updating the docs to take this in to account.  Also sorted them to
match cmake while I was changing these lines.

Reviewed by Chris B.

git-svn-id: https://llvm.org/svn/llvm-project/llvm/trunk@244123 91177308-0d34-0410-b5e6-96231b3b80d8