tools: relocate-rela: Use global variables

Declare rela_start/end and text_base as global variables. It will help with
using these variables for ELF decoding.

Signed-off-by: Michal Simek <michal.simek@amd.com>
Link: https://lore.kernel.org/r/7485b163e92f8f3f754c35f7c88c3314f2212efd.1655299267.git.michal.simek@amd.com