[ARM] Implement PLT for FDPIC.

[ARM] Implement PLT for FDPIC.

FDPIC requires special PLT entries, defined in this patch.

Note that lazy binding is not supported because of a race condition
for lack of an atomic 64-bits load instruction.

2018-04-25  Christophe Lyon  <christophe.lyon@st.com>
	Mickaël Guêné  <mickael.guene@st.com>

	bfd/
	* elf32-arm.c (elf32_arm_fdpic_plt_entry): New.
	(elf32_arm_create_dynamic_sections): Handle FDPIC.
	(elf32_arm_allocate_plt_entry): Likewise.
	(elf32_arm_populate_plt_entry): Likewise.
	(elf32_arm_output_plt_map_1): Likewise.