When -z bndplt is used, we must use the .plt.bnd entry for IFUNC function
address.
bfd/
PR ld/21481
* elf64-x86-64.c (elf_x86_64_finish_dynamic_symbol): Use .plt.bnd
for IFUNC function address.
ld/
PR ld/21481
* testsuite/ld-x86-64/pr21481a.c: New file.
* testsuite/ld-x86-64/pr21481b.S: Likewise.
* testsuite/ld-x86-64/x86-64.exp: Run PR ld/21481 tests.