gdbstub: Ignore indexing warning with -Warray-bounds

The current implementation can raise a warning as the compiler sees
an attempt to index an array with a size of zero. This can be fixed by
giving `gdb_mem_num_regions` a default value of zero, but this gets
flagged by CI checks. Disable the warning around the array access.

Signed-off-by: Robert Zieba <robertzieba@google.com>