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

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

When building with clang 18, I see:

      CXX    aarch64-linux-tdep.o
    /home/smarchi/src/binutils-gdb/gdb/aarch64-linux-tdep.c:1299:26: error: variable length arrays in C++ are a Clang extension [-Werror,-Wvla-cxx-extension]
     1299 |       gdb_byte za_zeroed[za_bytes];
          |                          ^~~~~~~~
    /home/smarchi/src/binutils-gdb/gdb/aarch64-linux-tdep.c:1299:26: note: read of non-const variable 'za_bytes' is not allowed in a constant expression
    /home/smarchi/src/binutils-gdb/gdb/aarch64-linux-tdep.c:1282:10: note: declared here
     1282 |   size_t za_bytes = std::pow (sve_vl_from_vg (svg), 2);
          |          ^

Since we are using VLAs right now, that warning doesn't make sense for
us.  add `-Wno-vla-cxx-extension` to the list of warning flags we try to
enable.  If we ever choose to disallow VLAs, we can remove that flag.

Change-Id: Ie41feafc50c343f6e75333d4f836ce32fbeb6d8c