gdb, gdbserver, gdbsupport: remove -Wno-vla-cxx-extension

Now that all known uses of VLAs within GDB are removed, remove the
`-Wno-vla-cxx-extension` (which was used to silence clang warnings) and
add `-Wvla`, such that any use of a VLA will trigger a warning.

Change-Id: I69a8d7f93f973743165b0ba46f9c2ea8adb89025