[ARM] FDPIC: Implement Thumb-only PLT for FDPIC.

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

	bfd/
	* elf32-arm.c (elf32_arm_fdpic_thumb_plt_entry): New.
	(elf32_arm_plt_needs_thumb_stub_p): Handle thumb-only case.
	(elf32_arm_populate_plt_entry): Likewise.
	(elf32_arm_output_plt_map_1): Likewise.
	(elf32_arm_output_arch_local_syms): Likewise.

	ld/testsuite/
	* arm-elf.exp: Execute the new FDPIC Thumb-only tests.
	* fdpic-main-m.d: New test.
	* fdpic-main-m.s: New.
	* fdpic-main-m.sym: New.
	* fdpic-shared-m.d: New test.
	* fdpic-shared-m.s: New.
	* fdpic-shared-m.sym: New.