PR30536, ppc64el gold linker produces unusable clang-16 binary

In commit 0961e631, the fix for PR30217, make_lplt_section and
make_brlt_section were changed to use rela_dyn_ rather than their own
separate dynamic reloc sections.  This fails miserably whenever brlt_
is needed for long branches, due to needing to iterate sizing and thus
reset brlt_ sizes.

	PR 30536
	PR 30217
	* powerpc.cc (Target_powerpc::make_brlt_section): Don't use
	rela_dyn_.

(cherry picked from commit dc808a2e)