[libc++] Move the GDB pretty printer tests to the DSL

Also, enable them whenever we detect that gdb is available. Previously,
these tests would basically never run because they relied on a CMake
configuration option that defaulted to OFF.

Differential Revision: https://reviews.llvm.org/D91434

GitOrigin-RevId: 7ad8e19958b271b9369e6b9d8d76f2f9f1beeb90
8 files changed