RISC-V: Add RV32E support.

RISC-V: Add RV32E support.

	Kito Cheng  <kito.cheng@gmail.com>
	Monk Chiang  <sh.chiang04@gmail.com>

	bfd/
	* elfnn-riscv.c (_bfd_riscv_elf_merge_private_bfd_data): Handle
	EF_RISCV_RVE.

	binutils/
	* readelf.c (get_machine_flags): Handle EF_RISCV_RVE.

	gas/
	* config/tc-riscv.c (rve_abi): New.
	(riscv_set_options): Add rve field.  Initialize it.
	(riscv_set_rve) New function.
	(riscv_set_arch): Support 'e' ISA subset.
	(reg_lookup_internal): If rve, check register is available.
	(riscv_set_abi): New parameter rve.
	(md_parse_option): Pass new argument to riscv_set_abi.
	(riscv_after_parse_args): Call riscv_set_rve.  If rve_abi, set
	EF_RISCV_RVE.
	* doc/c-riscv.texi (-mabi): Document new ilp32e argument.

	include/
	* elf/riscv.h (EF_RISCV_RVE): New define.