ARM: Fix exidx coverage for relocatable builds.

ARM: Fix exidx coverage for relocatable builds.

bfd  * elf-bfd.h: Add callback to count additional relocations.
     * elf32-arm.c (_arm_elf_section_data): Add new counter.
     (insert_cantunwind_after): Increment relocations counter.
     (elf32_arm_fix_exidx_coverage): Remove exidx entries and add
     terminating CANTUNWIND entry only in final builds.
     (elf32_arm_add_relocation): New function.
     (elf32_arm_write_section): Add relocations in relocatable builds.
     (elf32_arm_count_additional_relocs): New function.
     (elf_backend_count_additional_relocs): New define.
     * bfd/elflink.c (bfd_elf_final_link): Use callback and adjust size of
     .rel section.
     * bfd/elfxx-target.h (elf_backend_count_additional_relocs): New define.

ld   * emultempl/armelf.em (gld${EMULATION_NAME}_after_allocation): Call
     elf32_arm_fix_exidx_coverage for relocatable builds.

ld/testsuite
     * ld-arm/arm-elf.exp: New test.
     * ld-arm/unwind-rel.d: New file.
     * ld-arm/unwind-rel1.s: New file.
     * ld-arm/unwind-rel2.s: New file.
     * ld-arm/unwind-rel3.s: New file.