This commit introduces tools/update_glibc.zig to update the start files
for next time.

Some notable changes in recent glibc:

 * abi-note.S has been changed to abi-note.c but we resist the change to
   keep it easier to compile the start files.
 * elf-init.c has been deleted upstream. Further testing should be done
   to verify that binaries against glibc omitting elf-init.c still run
   properly on oldel glibc linux systems.

Closes #4926