RISC-V: Initial RV64E and LP64E support

RISC-V: Initial RV64E and LP64E support

Along with RV32E, RV64E is ratified.  Though ILP32E and LP64E ABIs are
still draft, it's worth supporting it.

gcc/ChangeLog:

	* common/config/riscv/riscv-common.cc
	(riscv_ext_version_table): Set version to ratified 2.0.
	(riscv_subset_list::parse_std_ext): Allow RV64E.
	* config.gcc: Parse base ISA 'rv64e' and ABI 'lp64e'.
	* config/riscv/arch-canonicalize: Parse base ISA 'rv64e'.
	* config/riscv/riscv-c.cc (riscv_cpu_cpp_builtins):
	Define different macro per XLEN.  Add handling for ABI_LP64E.
	* config/riscv/riscv-d.cc (riscv_d_handle_target_float_abi):
	Add handling for ABI_LP64E.
	* config/riscv/riscv-opts.h (enum riscv_abi_type): Add ABI_LP64E.
	* config/riscv/riscv.cc (riscv_option_override): Enhance error
	handling to support RV64E and LP64E.
	(riscv_conditional_register_usage): Change "RV32E" in a comment
	to "RV32E/RV64E".
	* config/riscv/riscv.h
	(UNITS_PER_FP_ARG): Add handling for ABI_LP64E.
	(STACK_BOUNDARY): Ditto.
	(ABI_STACK_BOUNDARY): Ditto.
	(MAX_ARGS_IN_REGISTERS): Ditto.
	(ABI_SPEC): Add support for "lp64e".
	* config/riscv/riscv.opt: Parse -mabi=lp64e as ABI_LP64E.
	* doc/invoke.texi: Add documentation of the LP64E ABI.

gcc/testsuite/ChangeLog:

	* gcc.target/riscv/predef-1.c: Test for __riscv_64e.
	* gcc.target/riscv/predef-2.c: Ditto.
	* gcc.target/riscv/predef-3.c: Ditto.
	* gcc.target/riscv/predef-4.c: Ditto.
	* gcc.target/riscv/predef-5.c: Ditto.
	* gcc.target/riscv/predef-6.c: Ditto.
	* gcc.target/riscv/predef-7.c: Ditto.
	* gcc.target/riscv/predef-8.c: Ditto.
	* gcc.target/riscv/predef-9.c: New test for RV64E and LP64E,
	based on predef-7.c.