debuginfo: Split debuginfo autotests into debuginfo-gdb and debuginfo-lldb
auto merge of #13726 : michaelwoerister/rust/lldb-autotests, r=alexcrichton

This pull request contains preparations for adding LLDB autotests:
+ the debuginfo tests are split into debuginfo-gdb and debuginfo-lldb
  + the `compiletest` tool is updated to support the debuginfo-lldb mode
  + tests.mk is modified to provide debuginfo-gdb and debuginfo-lldb make targets
  + GDB test cases are moved from `src/test/debug-info` to `src/test/debuginfo-gdb`
+ configure will now look for LLDB and set the appropriate CFG variables
+ the `lldb_batchmode.py` script is added to `src/etc`. It emulates GDB's batch mode

The LLDB autotests themselves are not part of this PR. Those will probable require some manual work on the test bots to make them work for the first time. Better to get these unproblematic preliminaries out of the way in a separate step.