[ARM] Add FDPIC OSABI flag support.

[ARM] Add FDPIC OSABI flag support.

ELF files targetting ARM FDPIC use the ELFOSABI_ARM_FDPIC flag.
Set it appropriately in file generators (eg. gas), and handle it in
readers (eg. readelf).

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

	bfd/
	* elf32-arm.c (elf32_arm_print_private_bfd_data): Support
	EF_ARM_PIC and ELFOSABI_ARM_FDPIC.
	(elf32_arm_post_process_headers): Support ELFOSABI_ARM_FDPIC.
	(ELF_OSABI): Define to ELFOSABI_ARM_FDPIC.

	binutils/
	* readelf.c (decode_ARM_machine_flags): Support EF_ARM_PIC.
	(get_osabi_name): Support ELFOSABI_ARM_FDPIC.

	gas/
	* config/tc-arm.c (arm_fdpic): New.
	(elf32_arm_target_format): Support FDPIC.
	(OPTION_FDPIC): New.
	(md_longopts): Support FDPIC.
	(md_parse_option): Likewise.
	(md_show_usage): Likewise.

	include/
	* elf/arm.h (EF_ARM_FDPIC): New.