bfd: xtensa: fix shrink_dynamic_reloc_sections for export-dynamic

bfd: xtensa: fix shrink_dynamic_reloc_sections for export-dynamic

shrink_dynamic_reloc_sections must remove PLT entry that was created for
an undefined weak symbol in the presence of --export-dynamic option when
relaxation coalesces literals pointing to that symbol. This fixes the
following assertion:

  ld: BFD (GNU Binutils) 2.31.1 internal error, aborting at
  elf32-xtensa.c:3292 in elf_xtensa_finish_dynamic_sections

2019-03-29  Max Filippov  <jcmvbkbc@gmail.com>
bfd/
	* elf32-xtensa.c (shrink_dynamic_reloc_sections): Add
	info->export_dynamic to the conditional.

ld/
	* testsuite/ld-xtensa/relax-undef-weak-pie-export-dynamic.d: New
	test definition.
	* testsuite/ld-xtensa/xtensa.exp
	(relax-undef-weak-pie-export-dynamic): Add new test.