[cross-project-tests] XFAIL llgdb-tests when gdb can't read clang's DWARF

Tests in the `cross-project-tests/debuginfo-tests/llgdb-tests` directory run
gdb on non-darwin platforms. gdb versions less than 10.1 cannot parse the DWARF
v5 emitted by clang, and DWARF v5 is now the default, so these tests fail on
Linux with gdb versions less than 10.1. This patch lets us XFAIL the tests
under these conditions.

Add `gdb-clang-incompatibility` to the `available_features` in
`cross-project-tests/lit.cfg.py` when clang's default DWARF version is 5 or
greater and the gdb (if found) version is less than 10.1.

Discourse discussion:
https://llvm.discourse.group/t/gdb-10-1-cant-read-clangs-dwarf-v5/6035

Reviewed By: jmorse

Differential Revision: https://reviews.llvm.org/D118468