* elf32-arm.c (elf32_arm_finish_dynamic_sections): Report an error
if a required section is missing from the linker script.
* ld-arm/arm-elf.exp: Add tests of linking without .rel.plt.
* ld-arm/arm-no-rel-plt.out: Linker script without .rel.plt section.
* ld-arm/arm-no-rel-plt.out: Expected output.
