[ARM] Add TLS relocations for FDPIC.

[ARM] Add TLS relocations for FDPIC.

Define and handle TLS relocations for FDPIC in BFD and gas.

In gas, the new relocations are rejected if the --fdpic option was not
specified.

We also define the __tdata_start symbol to mark the start of the
.tdata section. This allows FDPIC static binaries to find the start of
.tdata section, since phdr->p_vaddr of TLS segment is not a valid
value for FDPIC.

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

	bfd/:
	* bfd-in2.h (BFD_RELOC_ARM_TLS_GD32_FDPIC)
	(BFD_RELOC_ARM_TLS_LDM32_FDPIC, BFD_RELOC_ARM_TLS_IE32_FDPIC): New
	relocations.
	* elf32-arm.c (elf32_arm_howto_table_2): Add R_ARM_TLS_GD32_FDPIC,
	R_ARM_TLS_LDM32_FDPIC, R_ARM_TLS_IE32_FDPIC relocations.
	(elf32_arm_reloc_map): Add R_ARM_TLS_GD32_FDPIC,
	R_ARM_TLS_LDM32_FDPIC, R_ARM_TLS_IE32_FDPIC.
	(struct elf32_arm_link_hash_table): Update comment.
	(elf32_arm_final_link_relocate): Handle TLS FDPIC relocations.
	(IS_ARM_TLS_RELOC): Likewise.
	(elf32_arm_check_relocs): Likewise.
	(allocate_dynrelocs_for_symbol): Likewise.
	(elf32_arm_size_dynamic_sections): Update comment.
	* reloc.c: Add BFD_RELOC_ARM_TLS_GD32_FDPIC,
	BFD_RELOC_ARM_TLS_LDM32_FDPIC, BFD_RELOC_ARM_TLS_IE32_FDPIC.

	gas/
	* config/tc-arm.c (reloc_names): Add TLSGD_FDPIC, TLSLDM_FDPIC,
	GOTTPOFF_FDIC relocations.
	(md_apply_fix): Handle the new TLS FDPIC relocations.
	(tc_gen_reloc): Likewise.
	(arm_fix_adjustable): Likewise.

	include/
	* elf/arm.h: Add R_ARM_TLS_GD32_FDPIC, R_ARM_TLS_LDM32_FDPIC,
	R_ARM_TLS_IE32_FDPIC.

	ld/
	* scripttempl/elf.sc: Define __tdata_start for .tdata section.